#include <bits/stdc++.h>
template <class T>
const T& min(const T& a, const T& b, const T& c) {
  return min(a, min(b, c));
}
template <class T>
const T& max(const T& a, const T& b, const T& c) {
  return max(a, max(b, c));
}
using namespace std;
int gcd(long long a, long long b) {
  while (true) {
    if (a < b) swap(a, b);
    if (b == 0) return a;
    a = a % b;
  }
}
int main() {
  int n, k;
  cin >> n >> k;
  if (k > n) {
    cout << "-1";
    return 0;
  }
  if (k == 1) {
    if (n == 1)
      cout << "a";
    else
      cout << "-1";
    return 0;
  }
  if (k == 2) {
    for (__typeof(n) i = 0; i < n; i++) {
      cout << (((i % 2) == 0) ? "a" : "b");
    }
    return 0;
  }
  int written = 0;
  for (__typeof(n - (k - 2)) i = 0; i < n - (k - 2); i++) {
    cout << (((i % 2) == 0) ? 'a' : 'b');
  }
  for (__typeof(k - 2) i = 0; i < k - 2; i++) {
    char c = 'c' + i;
    cout << c;
  }
}
template <class T>
string toStr(T t) {
  std::stringstream strstream;
  string resStr;
  strstream << t;
  strstream >> resStr;
  return resStr;
}

### Prompt

Generate a CPP solution to the following problem:

Little penguin Polo adores strings. But most of all he adores strings of length n.

One day he wanted to find a string that meets the following conditions:

  1. The string consists of n lowercase English letters (that is, the string's length equals n), exactly k of these letters are distinct. 
  2. No two neighbouring letters of a string coincide; that is, if we represent a string as s = s1s2... sn, then the following inequality holds, si ≠ si + 1(1 ≤ i < n). 
  3. Among all strings that meet points 1 and 2, the required string is lexicographically smallest. 



Help him find such string or state that such string doesn't exist.

String x = x1x2... xp is lexicographically less than string y = y1y2... yq, if either p < q and x1 = y1, x2 = y2, ... , xp = yp, or there is such number r (r < p, r < q), that x1 = y1, x2 = y2, ... , xr = yr and xr + 1 < yr + 1. The characters of the strings are compared by their ASCII codes.

Input

A single line contains two positive integers n and k (1 ≤ n ≤ 106, 1 ≤ k ≤ 26) — the string's length and the number of distinct letters.

Output

In a single line print the required string. If there isn't such string, print "-1" (without the quotes).

Examples

Input

7 4


Output

ababacd


Input

4 7


Output

-1

### Response

```cpp
#include <bits/stdc++.h>
template <class T>
const T& min(const T& a, const T& b, const T& c) {
  return min(a, min(b, c));
}
template <class T>
const T& max(const T& a, const T& b, const T& c) {
  return max(a, max(b, c));
}
using namespace std;
int gcd(long long a, long long b) {
  while (true) {
    if (a < b) swap(a, b);
    if (b == 0) return a;
    a = a % b;
  }
}
int main() {
  int n, k;
  cin >> n >> k;
  if (k > n) {
    cout << "-1";
    return 0;
  }
  if (k == 1) {
    if (n == 1)
      cout << "a";
    else
      cout << "-1";
    return 0;
  }
  if (k == 2) {
    for (__typeof(n) i = 0; i < n; i++) {
      cout << (((i % 2) == 0) ? "a" : "b");
    }
    return 0;
  }
  int written = 0;
  for (__typeof(n - (k - 2)) i = 0; i < n - (k - 2); i++) {
    cout << (((i % 2) == 0) ? 'a' : 'b');
  }
  for (__typeof(k - 2) i = 0; i < k - 2; i++) {
    char c = 'c' + i;
    cout << c;
  }
}
template <class T>
string toStr(T t) {
  std::stringstream strstream;
  string resStr;
  strstream << t;
  strstream >> resStr;
  return resStr;
}

```