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>
int n, t;
std::string a, b, c = "";
bool visited[100000];
int main() {
  std::cin >> n >> t >> a >> b;
  c.resize(n, ' ');
  int rem_a = n - t, rem_b = n - t;
  for (int i = 0; i < n; i++) {
    if (a[i] != b[i]) {
      continue;
    }
    if (rem_a == 0 || rem_b == 0) break;
    c[i] = a[i];
    rem_a--;
    rem_b--;
    visited[i] = true;
  }
  for (int i = 0; i < n; i++) {
    if (visited[i]) continue;
    char ch;
    if (rem_a > 0 && rem_b > 0) {
      ch = a[i];
      rem_a--;
      if (b[i] == a[i]) rem_b--;
    } else if (rem_a > 0) {
      ch = a[i];
      if (b[i] == ch) ch++;
    } else if (rem_b > 0) {
      ch = b[i];
      rem_b--;
      if (a[i] == ch) ch++;
    } else {
      for (int k = 'a'; k <= 'z'; k++)
        if (k != a[i] && k != b[i]) {
          ch = k;
          break;
        }
    }
    c[i] = ch;
  }
  std::cout << (rem_a == 0 && rem_b == 0 ? c : "-1");
  return 0;
}