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;
int n, t, t1, t2;
string a, b;
char ans[100000];
char f1(char a1, char a2) {
  if ('a' != a1 && 'a' != a2) return 'a';
  if ('b' != a1 && 'b' != a2) return 'b';
  return 'c';
}
int main() {
  cin >> n >> t >> a >> b;
  t = n - t;
  for (int i = 0; i < n; i++) {
    if (a[i] == b[i] && t > 0) {
      t--;
      ans[i] = a[i];
    }
  }
  t1 = t2 = t;
  for (int i = 0; i < n; i++) {
    if (ans[i] == 0) {
      if (t1 > t2) {
        ans[i] = a[i];
        t1--;
      } else if (t2 > t1) {
        ans[i] = b[i];
        t2--;
      } else if (t1 != 0) {
        ans[i] = b[i];
        t2--;
      } else {
        ans[i] = f1(a[i], b[i]);
      }
    }
  }
  if (t1 == 0 && t2 == 0) {
    for (int i = 0; i < n; i++) {
      cout << ans[i];
    }
  } else {
    cout << -1;
  }
}