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 s[100010];
int main() {
  int n, t;
  cin >> n >> t;
  string s1, s2;
  cin >> s1 >> s2;
  for (int i = 0; i < n; ++i) {
    for (char j = 'a'; j <= 'z'; ++j) {
      if (s1[i] != j and s2[i] != j) {
        s[i] = j;
        break;
      }
    }
  }
  int same2;
  int same1 = same2 = n - t;
  for (int i = 0; i < n; ++i) {
    if (!same1) break;
    if (s1[i] == s2[i]) {
      s[i] = s1[i];
      same1--;
      same2--;
    }
  }
  for (int i = 0; i < n; ++i) {
    if (!same2) break;
    if (s1[i] != s2[i]) {
      if (same1 >= same2) {
        same1--;
        s[i] = s1[i];
      } else {
        same2--;
        s[i] = s2[i];
      }
    }
  }
  if (same1 or same2)
    cout << -1;
  else
    cout << s;
  return 0;
}