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 s1[110000], s2[110000];
int main() {
  int n, t;
  scanf("%d%d%s%s", &n, &t, s1, s2);
  int sam = 0;
  int dif = 0;
  t = n - t;
  for (int j = 0; j < n; ++j) {
    if (s1[j] == s2[j])
      ++sam;
    else
      ++dif;
  }
  if (sam + dif / 2 < t)
    printf("-1\n");
  else {
    int db = 0, sb = 0;
    bool flag = 0;
    if (t > sam)
      db = t - sam;
    else
      sb = sam - t;
    for (int j = 0; j < n; ++j) {
      if (s1[j] != s2[j]) {
        if (db) {
          if (!flag) {
            printf("%c", s1[j]);
          } else {
            printf("%c", s2[j]);
            --db;
          }
          flag = !flag;
        } else {
          char i = 'a';
          while (i == s1[j] || i == s2[j]) ++i;
          printf("%c", i);
        }
      } else {
        if (sb) {
          char i = 'a';
          while (i == s1[j]) ++i;
          printf("%c", i);
          --sb;
        } else
          printf("%c", s1[j]);
      }
    }
    printf("\n");
  }
  return 0;
}