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 = 1e9 + 7;
int n, t;
char s1[100009], s2[100009];
char s3[100009];
char findDif(char c1, char c2) {
  for (char x = 'a'; x <= 'z'; ++x)
    if (x != c1 && x != c2) return x;
}
int main() {
  cin >> n >> t;
  t = n - t;
  cin >> s1 >> s2;
  int cnt = 0;
  for (int i = 0; i < (n); ++i) cnt += (s1[i] == s2[i]);
  if (cnt >= t) {
    cnt = 0;
    for (int i = 0; i < (n); ++i)
      if (s1[i] == s2[i] && cnt < t) {
        ++cnt;
        s3[i] = s1[i];
      } else
        s3[i] = findDif(s1[i], s2[i]);
    s3[n] = 0;
    cout << s3 << endl;
    return 0;
  }
  int sm1 = t - cnt;
  int sm2 = t - cnt;
  for (int i = 0; i < (n); ++i)
    if (s1[i] == s2[i])
      s3[i] = s1[i];
    else if (sm1) {
      --sm1;
      s3[i] = s1[i];
    } else if (sm2) {
      --sm2;
      s3[i] = s2[i];
    } else
      s3[i] = findDif(s1[i], s2[i]);
  if (sm1 || sm2) {
    puts("-1");
    return 0;
  }
  s3[n] = 0;
  cout << s3 << endl;
  return 0;
}