#include <bits/stdc++.h>
using namespace std;
FILE* _fin = stdin;
FILE* _fout = stdout;
int _min(int a, int b) { return a <= b ? a : b; }
int _min(long long a, long long b) { return a <= b ? a : b; }
int _max(int a, int b) { return a >= b ? a : b; }
long long _max(long long a, long long b) { return a >= b ? a : b; }
void zero(int* data, int n) { memset(data, 0, sizeof(int) * n); }
void zero(long long* data, int n) { memset(data, 0, sizeof(long long) * n); }
void zero(unsigned long long* data, int n) {
  memset(data, 0, sizeof(unsigned long long) * n);
}
void zero(char* data, int n) { memset(data, 0, sizeof(char) * n); }
char readc() {
  char var;
  fscanf(_fin, "%c", &var);
  return var;
}
int readi() {
  int var;
  fscanf(_fin, "%d", &var);
  return var;
}
double readlf() {
  double var;
  fscanf(_fin, "%lf", &var);
  return var;
}
long long readll() {
  long long var;
  fscanf(_fin, "%lld", &var);
  return var;
}
void repread(int* data, int n) {
  for (int i = 0; i < n; ++i) data[i] = readi();
}
void repread(long long* data, int n) {
  for (int i = 0; i < n; ++i) data[i] = readll();
}
int reads(char* str, int maxsize) {
  for (;;) {
    if (fgets(str, maxsize, _fin) == NULL) break;
    if (str[0] != '\n' && str[0] != '\r') break;
  }
  int slen = strlen(str);
  if (slen == 0) return 0;
  if (str[slen - 1] == '\n' || str[slen - 1] == '\r') str[--slen] = 0;
  return slen;
}
long long gcd(long long a, long long b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
long long lcm(long long a, long long b) {
  long long g = gcd(a, b);
  return (a / g) * b;
}
void reverse(char* data, int n) {
  int k = n >> 1;
  for (int i = 0; i < k; ++i) {
    char tmp = data[i];
    data[i] = data[n - i - 1];
    data[n - i - 1] = tmp;
  }
}
void reverse(int* data, int n) {
  int k = n >> 1;
  for (int i = 0; i < k; ++i) {
    int tmp = data[i];
    data[i] = data[n - i - 1];
    data[n - i - 1] = tmp;
  }
}
void reverse(long long* data, int n) {
  int k = n >> 1;
  for (int i = 0; i < k; ++i) {
    long long tmp = data[i];
    data[i] = data[n - i - 1];
    data[n - i - 1] = tmp;
  }
}
struct Veci {
  int* data;
  int size;
  int n;
};
void init(Veci* t, int size) {
  t->data = (int*)malloc(sizeof(int) * size);
  t->size = size;
  t->n = 0;
}
void resize(Veci* t) {
  int ns = t->size * 1.2f;
  t->data = (int*)realloc(t->data, sizeof(int) * ns);
  t->size = ns;
}
void add(Veci* t, int val) {
  if (t->n >= t->size) resize(t);
  int k = t->n;
  t->data[k] = val;
  t->n = k + 1;
}
void free(Veci* t) { free(t->data); }
struct Vecll {
  long long* data;
  int size;
  int n;
};
void init(Vecll* t, int size) {
  t->data = (long long*)malloc(sizeof(long long) * size);
  t->size = size;
  t->n = 0;
}
void resize(Vecll* t) {
  int ns = t->size * 1.2f;
  t->data = (long long*)realloc(t->data, sizeof(long long) * ns);
  t->size = ns;
}
void add(Vecll* t, long long val) {
  if (t->n >= t->size) resize(t);
  int k = t->n;
  t->data[k] = val;
  t->n = k + 1;
}
void free(Vecll* t) { free(t->data); }
struct Vecs {
  char** data;
  int size;
  int n;
};
void init(Vecs* t, int size) {
  t->data = (char**)malloc(sizeof(char*) * size);
  t->size = size;
  t->n = 0;
}
void resize(Vecs* t) {
  int ns = t->size * 1.2f;
  t->data = (char**)realloc(t->data, sizeof(char*) * ns);
  t->size = ns;
}
void add(Vecs* t, char* val) {
  if (t->n >= t->size) resize(t);
  int k = t->n;
  t->data[k] = val;
  t->n = k + 1;
}
void free(Vecs* t) { free(t->data); }
int ispali(int* a, int* b, int n) {
  for (int i = 0; i < n; ++i) {
    if (a[i] != b[n - i - 1]) {
      return 0;
    }
  }
  return 1;
}
int ispalc(char* a, char* b, int n) {
  for (int i = 0; i < n; ++i) {
    if (a[i] != b[n - i - 1]) {
      return 0;
    }
  }
  return 1;
}
struct Pairi {
  int x, y;
};
int cmp_Pairi(const void* _a, const void* _b) {
  Pairi* a = (Pairi*)_a;
  Pairi* b = (Pairi*)_b;
  if (a->x < b->x)
    return -1;
  else {
    if (a->x == b->x) {
      if (a->y == b->y) return 0;
      if (a->y < b->y)
        return -1;
      else
        return 1;
    } else {
      return 1;
    }
  }
}
void sort_Pairi(Pairi* d, int n) { qsort(d, n, sizeof(Pairi), cmp_Pairi); }
struct Pairlf {
  double x, y;
};
int cmp_Pairlf(const void* _a, const void* _b) {
  Pairlf* a = (Pairlf*)_a;
  Pairlf* b = (Pairlf*)_b;
  if (a->x < b->x)
    return -1;
  else {
    if (a->x == b->x) {
      if (a->y == b->y) return 0;
      if (a->y < b->y)
        return -1;
      else
        return 1;
    } else {
      return 1;
    }
  }
}
void sort_Pairlf(Pairlf* d, int n) { qsort(d, n, sizeof(Pairlf), cmp_Pairlf); }
int cmp_Str(const void* _a, const void* _b) {
  char* a = *((char**)_a);
  char* b = *((char**)_b);
  return strcmp(a, b);
}
void sort_Str(char** str, int n) { qsort(str, n, sizeof(char*), cmp_Str); }
long long expmod(long long x, long long n, long long m) {
  long long ans = 1;
  for (; n;) {
    if (n & 1) ans = (ans * x) % m;
    x = (x * x) % m;
    n >>= 1;
  }
  return ans;
}
long long combmod(long long n, long long k, long long m) {
  long long ret = 1;
  long long div = 1;
  for (long long i = 0; i < k; ++i) {
    ret = (ret * (n - i) % m) % m;
    div = (div * (i + 1)) % m;
  }
  div = expmod(div, m - 2, m) % m;
  return (ret * div) % m;
}
int next_perm(int* data, int n) {
  int rootidx = -1;
  for (int i = n - 2; i >= 0; --i) {
    if (data[i] < data[i + 1]) {
      rootidx = i;
      break;
    }
  }
  if (rootidx == -1) return 0;
  int ceilingidx = rootidx + 1;
  for (int i = rootidx + 2; i < n; ++i) {
    if (data[i] > data[rootidx] && data[i] < data[ceilingidx]) {
      ceilingidx = i;
    }
  }
  {
    int t = data[rootidx];
    data[rootidx] = data[ceilingidx];
    data[ceilingidx] = t;
  };
  std::sort(&data[rootidx + 1], &data[rootidx + 1] + n - rootidx - 1);
  return 1;
}
int count_digits(long long a) {
  int k = 0;
  for (;;) {
    if (a == 0) break;
    k++;
    a /= 10;
  }
  return k;
}
int bs_exists(int* data, int n, int val) {
  int left = 0;
  int right = n - 1;
  for (; left < right;) {
    int mid = (left + right + 1) >> 1;
    if (data[mid] <= val)
      left = mid;
    else
      right = mid - 1;
  }
  return right;
}
double simple_factorial(int x) {
  double f = 1.0;
  for (int i = 0; i < x; ++i) {
    f *= (i + 1);
  }
  return f;
}
unsigned long long djb2(char* data, int n) {
  unsigned long long h = 5381;
  for (int i = 0; i < n; ++i) h = (h << 5) + h + data[i];
  return h;
}
struct Hashcount {
  int* count;
  unsigned long long* keys;
  int size_bits;
};
void init(Hashcount* h, int size_bits) {
  h->size_bits = size_bits;
  int size = 1 << h->size_bits;
  h->count = (int*)malloc(sizeof(int) * size);
  zero(h->count, size);
  h->keys = (unsigned long long*)malloc(sizeof(unsigned long long) * size);
  zero(h->keys, size);
}
int set(Hashcount* h, unsigned long long key) {
  int index = (key * 2654435761) >> (64 - h->size_bits);
  int size = 1 << h->size_bits;
  for (int i = 0; i < size; i++) {
    if (h->keys[index] == key) {
      h->count[index]++;
      return index;
    }
    if (h->keys[index] == 0) {
      h->count[index] = 1;
      h->keys[index] = key;
      return index;
    }
    index++;
    index &= size - 1;
  }
  return -1;
}
int get(Hashcount* h, unsigned long long key) {
  int index = (key * 2654435761) >> (64 - h->size_bits);
  int size = 1 << h->size_bits;
  for (int i = 0; i < size; i++) {
    if (h->keys[index] == key) {
      return h->count[index];
    }
    if (h->keys[index] == 0) return 0;
    index++;
    index &= size - 1;
  }
  return 0;
}
int sub(Hashcount* h, unsigned long long key) {
  int index = (key * 2654435761) >> (64 - h->size_bits);
  int size = 1 << h->size_bits;
  for (int i = 0; i < size; i++) {
    if (h->keys[index] == key) {
      --h->count[index];
      if (h->count[index] < 0) h->count[index] = 0;
      return h->count[index];
    }
    if (h->keys[index] == 0) return 0;
    index++;
    index &= size - 1;
  }
  return 0;
}
const long long MOD = 1e9 + 7;
int main() {
  {
    _fin = fopen("input.txt", "r");
    if (_fin == NULL) _fin = stdin;
  };
  int n = readi();
  int m = readi();
  int best = -1;
  for (int s2 = n / 2; s2 >= 0; --s2) {
    int rem = n - s2 * 2;
    int nmoves = rem + s2;
    if (nmoves % m == 0) {
      best = nmoves;
      break;
    }
  }
  printf("%d\n", best);
  return 0;
}

### Prompt

Generate a Cpp solution to the following problem:

Dreamoon wants to climb up a stair of n steps. He can climb 1 or 2 steps at each move. Dreamoon wants the number of moves to be a multiple of an integer m. 

What is the minimal number of moves making him climb to the top of the stairs that satisfies his condition?

Input

The single line contains two space separated integers n, m (0 < n ≤ 10000, 1 < m ≤ 10).

Output

Print a single integer — the minimal number of moves being a multiple of m. If there is no way he can climb satisfying condition print  - 1 instead.

Examples

Input

10 2


Output

6


Input

3 5


Output

-1

Note

For the first sample, Dreamoon could climb in 6 moves with following sequence of steps: {2, 2, 2, 2, 1, 1}.

For the second sample, there are only three valid sequence of steps {2, 1}, {1, 2}, {1, 1, 1} with 2, 2, and 3 steps respectively. All these numbers are not multiples of 5.

### Response

```cpp
#include <bits/stdc++.h>
using namespace std;
FILE* _fin = stdin;
FILE* _fout = stdout;
int _min(int a, int b) { return a <= b ? a : b; }
int _min(long long a, long long b) { return a <= b ? a : b; }
int _max(int a, int b) { return a >= b ? a : b; }
long long _max(long long a, long long b) { return a >= b ? a : b; }
void zero(int* data, int n) { memset(data, 0, sizeof(int) * n); }
void zero(long long* data, int n) { memset(data, 0, sizeof(long long) * n); }
void zero(unsigned long long* data, int n) {
  memset(data, 0, sizeof(unsigned long long) * n);
}
void zero(char* data, int n) { memset(data, 0, sizeof(char) * n); }
char readc() {
  char var;
  fscanf(_fin, "%c", &var);
  return var;
}
int readi() {
  int var;
  fscanf(_fin, "%d", &var);
  return var;
}
double readlf() {
  double var;
  fscanf(_fin, "%lf", &var);
  return var;
}
long long readll() {
  long long var;
  fscanf(_fin, "%lld", &var);
  return var;
}
void repread(int* data, int n) {
  for (int i = 0; i < n; ++i) data[i] = readi();
}
void repread(long long* data, int n) {
  for (int i = 0; i < n; ++i) data[i] = readll();
}
int reads(char* str, int maxsize) {
  for (;;) {
    if (fgets(str, maxsize, _fin) == NULL) break;
    if (str[0] != '\n' && str[0] != '\r') break;
  }
  int slen = strlen(str);
  if (slen == 0) return 0;
  if (str[slen - 1] == '\n' || str[slen - 1] == '\r') str[--slen] = 0;
  return slen;
}
long long gcd(long long a, long long b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
long long lcm(long long a, long long b) {
  long long g = gcd(a, b);
  return (a / g) * b;
}
void reverse(char* data, int n) {
  int k = n >> 1;
  for (int i = 0; i < k; ++i) {
    char tmp = data[i];
    data[i] = data[n - i - 1];
    data[n - i - 1] = tmp;
  }
}
void reverse(int* data, int n) {
  int k = n >> 1;
  for (int i = 0; i < k; ++i) {
    int tmp = data[i];
    data[i] = data[n - i - 1];
    data[n - i - 1] = tmp;
  }
}
void reverse(long long* data, int n) {
  int k = n >> 1;
  for (int i = 0; i < k; ++i) {
    long long tmp = data[i];
    data[i] = data[n - i - 1];
    data[n - i - 1] = tmp;
  }
}
struct Veci {
  int* data;
  int size;
  int n;
};
void init(Veci* t, int size) {
  t->data = (int*)malloc(sizeof(int) * size);
  t->size = size;
  t->n = 0;
}
void resize(Veci* t) {
  int ns = t->size * 1.2f;
  t->data = (int*)realloc(t->data, sizeof(int) * ns);
  t->size = ns;
}
void add(Veci* t, int val) {
  if (t->n >= t->size) resize(t);
  int k = t->n;
  t->data[k] = val;
  t->n = k + 1;
}
void free(Veci* t) { free(t->data); }
struct Vecll {
  long long* data;
  int size;
  int n;
};
void init(Vecll* t, int size) {
  t->data = (long long*)malloc(sizeof(long long) * size);
  t->size = size;
  t->n = 0;
}
void resize(Vecll* t) {
  int ns = t->size * 1.2f;
  t->data = (long long*)realloc(t->data, sizeof(long long) * ns);
  t->size = ns;
}
void add(Vecll* t, long long val) {
  if (t->n >= t->size) resize(t);
  int k = t->n;
  t->data[k] = val;
  t->n = k + 1;
}
void free(Vecll* t) { free(t->data); }
struct Vecs {
  char** data;
  int size;
  int n;
};
void init(Vecs* t, int size) {
  t->data = (char**)malloc(sizeof(char*) * size);
  t->size = size;
  t->n = 0;
}
void resize(Vecs* t) {
  int ns = t->size * 1.2f;
  t->data = (char**)realloc(t->data, sizeof(char*) * ns);
  t->size = ns;
}
void add(Vecs* t, char* val) {
  if (t->n >= t->size) resize(t);
  int k = t->n;
  t->data[k] = val;
  t->n = k + 1;
}
void free(Vecs* t) { free(t->data); }
int ispali(int* a, int* b, int n) {
  for (int i = 0; i < n; ++i) {
    if (a[i] != b[n - i - 1]) {
      return 0;
    }
  }
  return 1;
}
int ispalc(char* a, char* b, int n) {
  for (int i = 0; i < n; ++i) {
    if (a[i] != b[n - i - 1]) {
      return 0;
    }
  }
  return 1;
}
struct Pairi {
  int x, y;
};
int cmp_Pairi(const void* _a, const void* _b) {
  Pairi* a = (Pairi*)_a;
  Pairi* b = (Pairi*)_b;
  if (a->x < b->x)
    return -1;
  else {
    if (a->x == b->x) {
      if (a->y == b->y) return 0;
      if (a->y < b->y)
        return -1;
      else
        return 1;
    } else {
      return 1;
    }
  }
}
void sort_Pairi(Pairi* d, int n) { qsort(d, n, sizeof(Pairi), cmp_Pairi); }
struct Pairlf {
  double x, y;
};
int cmp_Pairlf(const void* _a, const void* _b) {
  Pairlf* a = (Pairlf*)_a;
  Pairlf* b = (Pairlf*)_b;
  if (a->x < b->x)
    return -1;
  else {
    if (a->x == b->x) {
      if (a->y == b->y) return 0;
      if (a->y < b->y)
        return -1;
      else
        return 1;
    } else {
      return 1;
    }
  }
}
void sort_Pairlf(Pairlf* d, int n) { qsort(d, n, sizeof(Pairlf), cmp_Pairlf); }
int cmp_Str(const void* _a, const void* _b) {
  char* a = *((char**)_a);
  char* b = *((char**)_b);
  return strcmp(a, b);
}
void sort_Str(char** str, int n) { qsort(str, n, sizeof(char*), cmp_Str); }
long long expmod(long long x, long long n, long long m) {
  long long ans = 1;
  for (; n;) {
    if (n & 1) ans = (ans * x) % m;
    x = (x * x) % m;
    n >>= 1;
  }
  return ans;
}
long long combmod(long long n, long long k, long long m) {
  long long ret = 1;
  long long div = 1;
  for (long long i = 0; i < k; ++i) {
    ret = (ret * (n - i) % m) % m;
    div = (div * (i + 1)) % m;
  }
  div = expmod(div, m - 2, m) % m;
  return (ret * div) % m;
}
int next_perm(int* data, int n) {
  int rootidx = -1;
  for (int i = n - 2; i >= 0; --i) {
    if (data[i] < data[i + 1]) {
      rootidx = i;
      break;
    }
  }
  if (rootidx == -1) return 0;
  int ceilingidx = rootidx + 1;
  for (int i = rootidx + 2; i < n; ++i) {
    if (data[i] > data[rootidx] && data[i] < data[ceilingidx]) {
      ceilingidx = i;
    }
  }
  {
    int t = data[rootidx];
    data[rootidx] = data[ceilingidx];
    data[ceilingidx] = t;
  };
  std::sort(&data[rootidx + 1], &data[rootidx + 1] + n - rootidx - 1);
  return 1;
}
int count_digits(long long a) {
  int k = 0;
  for (;;) {
    if (a == 0) break;
    k++;
    a /= 10;
  }
  return k;
}
int bs_exists(int* data, int n, int val) {
  int left = 0;
  int right = n - 1;
  for (; left < right;) {
    int mid = (left + right + 1) >> 1;
    if (data[mid] <= val)
      left = mid;
    else
      right = mid - 1;
  }
  return right;
}
double simple_factorial(int x) {
  double f = 1.0;
  for (int i = 0; i < x; ++i) {
    f *= (i + 1);
  }
  return f;
}
unsigned long long djb2(char* data, int n) {
  unsigned long long h = 5381;
  for (int i = 0; i < n; ++i) h = (h << 5) + h + data[i];
  return h;
}
struct Hashcount {
  int* count;
  unsigned long long* keys;
  int size_bits;
};
void init(Hashcount* h, int size_bits) {
  h->size_bits = size_bits;
  int size = 1 << h->size_bits;
  h->count = (int*)malloc(sizeof(int) * size);
  zero(h->count, size);
  h->keys = (unsigned long long*)malloc(sizeof(unsigned long long) * size);
  zero(h->keys, size);
}
int set(Hashcount* h, unsigned long long key) {
  int index = (key * 2654435761) >> (64 - h->size_bits);
  int size = 1 << h->size_bits;
  for (int i = 0; i < size; i++) {
    if (h->keys[index] == key) {
      h->count[index]++;
      return index;
    }
    if (h->keys[index] == 0) {
      h->count[index] = 1;
      h->keys[index] = key;
      return index;
    }
    index++;
    index &= size - 1;
  }
  return -1;
}
int get(Hashcount* h, unsigned long long key) {
  int index = (key * 2654435761) >> (64 - h->size_bits);
  int size = 1 << h->size_bits;
  for (int i = 0; i < size; i++) {
    if (h->keys[index] == key) {
      return h->count[index];
    }
    if (h->keys[index] == 0) return 0;
    index++;
    index &= size - 1;
  }
  return 0;
}
int sub(Hashcount* h, unsigned long long key) {
  int index = (key * 2654435761) >> (64 - h->size_bits);
  int size = 1 << h->size_bits;
  for (int i = 0; i < size; i++) {
    if (h->keys[index] == key) {
      --h->count[index];
      if (h->count[index] < 0) h->count[index] = 0;
      return h->count[index];
    }
    if (h->keys[index] == 0) return 0;
    index++;
    index &= size - 1;
  }
  return 0;
}
const long long MOD = 1e9 + 7;
int main() {
  {
    _fin = fopen("input.txt", "r");
    if (_fin == NULL) _fin = stdin;
  };
  int n = readi();
  int m = readi();
  int best = -1;
  for (int s2 = n / 2; s2 >= 0; --s2) {
    int rem = n - s2 * 2;
    int nmoves = rem + s2;
    if (nmoves % m == 0) {
      best = nmoves;
      break;
    }
  }
  printf("%d\n", best);
  return 0;
}

```