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 n, k, len;
string s1, s2;
char diff(char c1, char c2) {
  for (char c = 'a'; c <= 'z'; c++)
    if (c != c1 && c != c2) return c;
}
int main() {
  cin >> n >> k;
  getline(cin, s1, '\n');
  getline(cin, s1, '\n');
  getline(cin, s2, '\n');
  for (int i = 0; i <= n - 1; i++) len += (s1[i] != s2[i]);
  if (len == k) {
    for (int i = 0; i <= n - 1; i++)
      if (s1[i] == s2[i])
        cout << s1[i];
      else
        cout << diff(s1[i], s2[i]);
  } else if (len < k) {
    int dem = len;
    for (int i = 0; i <= n - 1; i++)
      if (s1[i] != s2[i])
        cout << diff(s1[i], s2[i]);
      else if (dem < k) {
        dem++;
        cout << diff(s1[i], s2[i]);
      } else
        cout << s1[i];
  } else {
    if (2 * k < len) {
      cout << -1;
      return 0;
    }
    int dem1 = 0, dem2 = 0;
    for (int i = 0; i <= n - 1; i++)
      if (s1[i] == s2[i])
        cout << s1[i];
      else if (dem1 < len - k) {
        cout << s1[i];
        dem1++;
      } else if (dem2 < len - k) {
        cout << s2[i];
        dem2++;
      } else
        cout << diff(s1[i], s2[i]);
  }
  return 0;
}