#include <bits/stdc++.h>
using namespace std;
constexpr int MAXN = 100000;
char s1[MAXN + 2], s2[MAXN + 2], ans[MAXN + 2];
char neither(char a, char b) {
  if (a > b) swap(a, b);
  char x = 'a';
  if (a == x) x++;
  if (b == x) x++;
  return x;
}
int main() {
  int n, t;
  cin >> n >> t >> s1 >> s2;
  t = n - t;
  for (int i = 0; i < n; i++) {
    if (s1[i] == s2[i]) {
      if (t) {
        ans[i] = s1[i];
        --t;
      } else {
        ans[i] = s1[i] == 'a' ? 'b' : 'a';
      }
    }
  }
  bool lst = false;
  for (int i = 0; i < n; i++) {
    if (s1[i] != s2[i]) {
      if (t) {
        if (!lst) {
          ans[i] = s1[i];
          lst ^= 1;
        } else {
          ans[i] = s2[i];
          lst ^= 1;
          --t;
        }
      } else {
        ans[i] = neither(s1[i], s2[i]);
      }
    }
  }
  if (t == 0)
    cout << ans << '\n';
  else
    cout << -1 << '\n';
}

### Prompt

Your task is to create a CPP solution to the following problem:

Marina loves strings of the same length and Vasya loves when there is a third string, different from them in exactly t characters. Help Vasya find at least one such string.

More formally, you are given two strings s1, s2 of length n and number t. Let's denote as f(a, b) the number of characters in which strings a and b are different. Then your task will be to find any string s3 of length n, such that f(s1, s3) = f(s2, s3) = t. If there is no such string, print  - 1.

Input

The first line contains two integers n and t (1 ≤ n ≤ 105, 0 ≤ t ≤ n).

The second line contains string s1 of length n, consisting of lowercase English letters.

The third line contain string s2 of length n, consisting of lowercase English letters.

Output

Print a string of length n, differing from string s1 and from s2 in exactly t characters. Your string should consist only from lowercase English letters. If such string doesn't exist, print -1.

Examples

Input

3 2
abc
xyc


Output

ayd

Input

1 0
c
b


Output

-1

### Response

```cpp
#include <bits/stdc++.h>
using namespace std;
constexpr int MAXN = 100000;
char s1[MAXN + 2], s2[MAXN + 2], ans[MAXN + 2];
char neither(char a, char b) {
  if (a > b) swap(a, b);
  char x = 'a';
  if (a == x) x++;
  if (b == x) x++;
  return x;
}
int main() {
  int n, t;
  cin >> n >> t >> s1 >> s2;
  t = n - t;
  for (int i = 0; i < n; i++) {
    if (s1[i] == s2[i]) {
      if (t) {
        ans[i] = s1[i];
        --t;
      } else {
        ans[i] = s1[i] == 'a' ? 'b' : 'a';
      }
    }
  }
  bool lst = false;
  for (int i = 0; i < n; i++) {
    if (s1[i] != s2[i]) {
      if (t) {
        if (!lst) {
          ans[i] = s1[i];
          lst ^= 1;
        } else {
          ans[i] = s2[i];
          lst ^= 1;
          --t;
        }
      } else {
        ans[i] = neither(s1[i], s2[i]);
      }
    }
  }
  if (t == 0)
    cout << ans << '\n';
  else
    cout << -1 << '\n';
}

```