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>
const int N = 1e5 + 1;
int n, t, i, pom, rowne, rozne, poma, pomb;
char a[N], b[N];
void rozny(int i) {
  if ('a' != a[i] && 'a' != b[i]) {
    printf("a");
  } else {
    if ('b' != a[i] && 'b' != b[i]) {
      printf("b");
    } else {
      printf("c");
    }
  }
}
int main() {
  scanf("%d%d", &n, &t);
  for (i = 0; i < n; i++) {
    scanf(" %c", &a[i]);
  }
  for (i = 0; i < n; i++) {
    scanf(" %c", &b[i]);
    if (a[i] == b[i]) {
      rowne++;
    } else {
      rozne++;
    }
  }
  if (rowne + (rozne / 2) < n - t) {
    printf("-1\n");
    return 0;
  }
  if (t >= rozne) {
    pom = t - rozne;
    for (i = 0; i < n; i++) {
      if (a[i] != b[i] || (a[i] == b[i] && pom > 0)) {
        if (a[i] == b[i]) {
          pom--;
        }
        rozny(i);
      } else {
        printf("%c", a[i]);
      }
    }
  } else {
    poma = rozne - t;
    pomb = poma;
    for (i = 0; i < n; i++) {
      if (a[i] == b[i]) {
        printf("%c", a[i]);
      } else {
        if (poma > 0) {
          printf("%c", a[i]);
          poma--;
        } else {
          if (pomb > 0) {
            printf("%c", b[i]);
            pomb--;
          } else {
            rozny(i);
          }
        }
      }
    }
  }
  printf("\n");
  return 0;
}