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;
void File() {
  freopen("in.txt", "r", stdin);
  freopen("out.txt", "w", stdout);
}
int main() {
  int n, t;
  cin >> n >> t;
  int match = n - t, mis = t;
  string s1, s2;
  cin >> s1 >> s2;
  int cnt = 0;
  for (int i = 0; i < n; ++i)
    if (s1[i] == s2[i]) cnt++;
  if (cnt >= match) {
    cnt = match;
    for (int i = 0; i < n; ++i) {
      if (cnt && s1[i] == s2[i]) {
        cout << s1[i];
        cnt--;
      } else {
        for (char j = 'a'; j <= 'z'; ++j) {
          if (j != s1[i] && j != s2[i]) {
            cout << j;
            break;
          }
        }
      }
    }
  } else if (cnt < match) {
    if ((match - cnt) * 2 + cnt > n) {
      puts("-1");
      return 0;
    }
    match -= cnt;
    match *= 2;
    bool bol = 0;
    for (int i = 0; i < n; ++i) {
      if (cnt && s1[i] == s2[i]) {
        cout << s1[i];
        cnt--;
      } else {
        if (match) {
          if (bol == 0) {
            cout << s1[i];
            bol = 1;
          } else {
            cout << s2[i];
            bol = 0;
          }
          match--;
        } else {
          for (char j = 'a'; j <= 'z'; ++j) {
            if (j != s1[i] && j != s2[i]) {
              cout << j;
              break;
            }
          }
        }
      }
    }
  }
  return 0;
}