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

Solution (in CPP):

Solution:
#include <bits/stdc++.h>
using namespace std;
char a[100010], b[100010], s[100010];
int f[100010];
char get(char c1, char c2) {
  char c = 'a';
  while (c <= 'z') {
    if (c != c1 && c != c2) return c;
    c++;
  }
}
int main() {
  int n, t;
  while (scanf("%d%d", &n, &t) != EOF) {
    scanf("%s%s", a, b);
    int ans = 0;
    for (int i = 0; i < n; i++) {
      if (a[i] == b[i]) {
        ans++;
        f[i] = 1;
      } else
        f[i] = 0;
    }
    int N = n - t;
    if (ans + 2 * (N - ans) > n) {
      puts("-1");
      continue;
    }
    for (int i = 0; i < n; i++) s[i] = get(a[i], b[i]);
    for (int i = 0; i < n; i++) {
      if (N == 0) break;
      if (f[i] == 1) {
        N--;
        f[i] = -1;
        s[i] = a[i];
      }
    }
    int k = 0;
    for (int i = 0; i < n; i++) {
      if (!N) break;
      if (f[i] == 0 && k <= N) {
        k++;
        s[i] = a[i];
      }
      if (k > N && k <= 2 * N && f[i] == 0) {
        k++;
        s[i] = b[i];
      }
    }
    for (int i = 0; i < n; i++) printf("%c", s[i]);
    printf("\n");
  }
  return 0;
}