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 dif(char u, char v) {
  for (int i = 97; i <= 97 + 26 - 1; i++) {
    char ch = (char)i;
    if ((ch != u) && (ch != v)) return ch;
  }
  return 'z';
}
int main() {
  int n, d = 0, k;
  string a, b, s = "";
  cin >> n >> k;
  cin >> a >> b;
  for (int i = 0; i <= n - 1; i++)
    if (a[i] != b[i]) d++;
  if (k >= d) {
    k -= d;
    for (int i = 0; i <= n - 1; i++) {
      if (a[i] != b[i])
        s += dif(a[i], b[i]);
      else if (k > 0) {
        k--;
        s += dif(a[i], b[i]);
      } else
        s += a[i];
    }
    if (k)
      cout << -1 << endl;
    else
      cout << s << endl;
  } else {
    int count = 0;
    for (int i = 0; i <= n - 1; i++)
      if (a[i] == b[i])
        s += a[i];
      else {
        if (k < d) {
          count = (count + 1) % 2;
          if (count == 0) {
            k++;
            s += b[i];
          } else
            s += a[i];
        } else
          s += dif(a[i], b[i]);
      }
    if (k < d)
      cout << -1 << endl;
    else
      cout << s << endl;
  }
  return 0;
}