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 str1[100005], str2[100005], str3[100005];
int main() {
  int n, m;
  while (scanf("%d %d", &n, &m) != EOF) {
    scanf("%s %s", str1, str2);
    int size = strlen(str1), num = 0;
    for (int i = 0; i < size; i++) {
      if (str1[i] == str2[i]) {
        if (num >= size - m)
          break;
        else
          num++;
      }
    }
    int diff = 0, same = num;
    int a = 0, b = 0;
    for (int i = 0; i < size; i++) {
      if (same && str1[i] == str2[i]) {
        same--;
        str3[i] = str1[i];
      } else {
        for (char j = 'a'; j <= 'z'; j++) {
          if (str1[i] == j || str2[i] == j)
            continue;
          else {
            if (a < size - m - num) {
              ++a;
              str3[i] = str1[i];
            } else if (b < size - m - num) {
              ++b;
              str3[i] = str2[i];
            } else {
              str3[i] = j;
            }
            break;
          }
        }
      }
    }
    a = 0;
    b = 0;
    for (int i = 0; i < size; i++) {
      if (str3[i] != str1[i]) a++;
      if (str3[i] != str2[i]) b++;
    }
    str3[n] = 0;
    if (a == m && b == m)
      printf("%s\n", str3);
    else
      printf("-1\n");
  }
  return 0;
}