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 m(char a, char b);
char c[100000];
int main() {
  int n, t;
  cin >> n >> t;
  string a, b;
  cin >> a >> b;
  int count, counter(-1);
  count = n - t;
  for (int i = 0; i < n; i++) {
    if (count == 0)
      c[i] = m(a[i], b[i]);
    else {
      if (a[i] == b[i]) {
        c[i] = a[i];
        count--;
      } else if (a[i] != b[i] && counter == -1) {
        counter = i;
        c[i] = m(a[i], b[i]);
      } else if (a[i] != b[i] && counter != -1) {
        c[i] = b[i];
        c[counter] = a[counter];
        counter = -1;
        count--;
      }
    }
  }
  if (count == 0) {
    puts(c);
  } else
    cout << "-1";
}
char m(char a, char b) {
  if (a == 'a' || b == 'a') {
    if (a == 'b' || b == 'b') {
      return 'c';
    } else
      return 'b';
  } else
    return 'a';
}