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 N = 1e5 + 10;
int n, t, cnt;
string s1, s2;
char s[N];
char diff(char a, char b) {
  if ('a' != a && 'a' != b) return 'a';
  if ('b' != a && 'b' != b) return 'b';
  return 'c';
}
int main() {
  cin >> n >> t >> s1 >> s2;
  for (int i = 0; i < n; i++)
    if (s1[i] != s2[i]) cnt++;
  if (t >= cnt) {
    int left = t - cnt;
    for (int i = 0; i < n; i++) {
      if (s1[i] != s2[i])
        s[i] = diff(s1[i], s2[i]);
      else {
        if (left) {
          s[i] = diff(s1[i], s2[i]);
          left--;
        } else
          s[i] = s1[i];
      }
    }
    printf("%s\n", s);
  } else if (t * 2 >= cnt) {
    int k = 0;
    for (int i = 0; i < n; i++) {
      if (s1[i] != s2[i]) {
        k++;
        if (k <= t * 2 - cnt)
          s[i] = diff(s1[i], s2[i]);
        else if (k <= t)
          s[i] = s2[i];
        else
          s[i] = s1[i];
      } else
        s[i] = s1[i];
    }
    printf("%s\n", s);
  } else {
    printf("-1\n");
  }
  return 0;
}