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 n, m;
  cin >> n >> m;
  string str, str1, str2(n, '0');
  cin >> str >> str1;
  if (m == 0) {
    if (str == str1) {
      cout << str << endl;
    } else
      cout << -1 << endl;
    return 0;
  }
  int cnt = 0;
  for (int i = 0; i < n; i++) {
    if (str[i] == str1[i])
      str2[i] = str[i];
    else {
      if (str[i] != 'm' && str1[i] != 'm')
        str2[i] = 'm';
      else if (str[i] != 'n' && str1[i] != 'n')
        str2[i] = 'n';
      else
        str2[i] = 'p';
      cnt++;
    }
  }
  if (cnt == m)
    cout << str2 << endl;
  else if (cnt > m) {
    int c = 0;
    for (int i = 0; i < n; i++) {
      if (str[i] != str1[i]) {
        c++;
        if (c % 2 == 1) {
          str2[i] = str[i];
        } else {
          str2[i] = str1[i];
          cnt--;
        }
        if (cnt == m) break;
      }
    }
    if (cnt == m)
      cout << str2 << endl;
    else
      cout << -1 << endl;
  } else {
    for (int i = 0; i < n; i++) {
      if (str[i] == str1[i]) {
        if (str[i] != 'm')
          str2[i] = 'm';
        else
          str2[i] = 'n';
        cnt++;
      }
      if (cnt == m) break;
    }
    cout << str2 << endl;
  }
  return 0;
}