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;
const int mod = 1000000007;
const int inf = 1001001001;
int n, t, k, a[1111111], ans;
string s1, s2, s3;
char G(char a, char b) {
  for (int i = 'a'; i <= 'z'; i++) {
    if (i != a && i != b) return i;
  }
}
int main() {
  cin >> n >> t >> s1 >> s2;
  int d = n - t;
  for (int i = 0; i < n; i++) {
    if (s1[i] == s2[i]) k++;
  }
  if (d <= k) {
    for (int i = 0; i < n; i++) {
      if (s1[i] != s2[i]) cout << G(s1[i], s2[i]);
      if (s1[i] == s2[i]) {
        if (d > 0) {
          d--;
          cout << s1[i];
        } else {
          cout << G(s1[i], s2[i]);
        }
      }
    }
  } else {
    if (k + (n - k) / 2 < d)
      cout << -1;
    else {
      int k2 = (d - k) * 2;
      int f = 0;
      for (int i = 0; i < n; i++) {
        if (d > 0 && s1[i] == s2[i]) {
          d--;
          cout << s1[i];
        }
        if (s1[i] != s2[i]) {
          if (k2 > 0) {
            if (f == 1) {
              cout << s1[i];
              f = 0;
              k2--;
            } else {
              cout << s2[i];
              f = 1;
              k2--;
            }
          } else {
            cout << G(s1[i], s2[i]);
          }
        }
      }
    }
  }
  return 0;
}