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 main() {
  int p1, p2;
  int n, t;
  string a, b, s = "";
  cin >> n >> t;
  cin >> a >> b;
  p1 = p2 = n - t;
  for (int i = 0; i < n; i++) s += '*';
  for (int i = 0; i < n; i++) {
    if (a[i] == b[i] && p1 > 0 && p2 > 0) {
      s[i] = a[i];
      p1--;
      p2--;
    }
  }
  for (int i = 0; i < n; i++) {
    if (s[i] == '*' && p1 > 0) {
      s[i] = a[i];
      p1--;
    } else if (s[i] == '*' && p2 > 0) {
      s[i] = b[i];
      p2--;
    }
  }
  if (p1 == 0 && p2 == 0) {
    for (int i = 0; i < n; i++) {
      if (s[i] == '*') {
        for (int j = 'a'; j <= 'z'; j++) {
          if (a[i] != char(j) && b[i] != char(j)) {
            s[i] = char(j);
            break;
          }
        }
      }
    }
    cout << s << endl;
  } else {
    cout << -1;
  }
}