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;
const int maxn = 1e5 + 10;
char s1[maxn], s2[maxn];
int n, t;
int main() {
  scanf("%d%d", &n, &t);
  scanf("%s%s", s1, s2);
  int len = strlen(s1);
  int dif = 0;
  for (int i = 0; i < len; i++)
    if (s1[i] != s2[i]) dif++;
  if (t >= dif) {
    int cnt = t - dif;
    for (int i = 0; i < len; i++) {
      if (s1[i] == s2[i]) {
        if (cnt) {
          for (int j = 0; j < 26; j++)
            if (j + 'a' != s1[i]) {
              putchar(j + 'a');
              break;
            }
          cnt--;
        } else
          putchar(s1[i]);
      } else {
        for (int j = 0; j < 26; j++)
          if (j + 'a' != s1[i] && j + 'a' != s2[i]) {
            putchar(j + 'a');
            break;
          }
      }
    }
  } else {
    int y = dif - t, x = 2 * t - dif;
    if (2 * y > dif)
      printf("-1\n");
    else {
      for (int i = 0; i < len; i++) {
        if (s1[i] == s2[i])
          putchar(s1[i]);
        else {
          if (x) {
            for (int j = 0; j < 26; j++)
              if (j + 'a' != s1[i] && j + 'a' != s2[i]) {
                putchar(j + 'a');
                break;
              }
            x--;
          } else if (y) {
            putchar(s1[i]);
            y--;
          } else {
            putchar(s2[i]);
          }
        }
      }
    }
  }
  return 0;
}