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 maxn = 1e5 + 10;
char s1[maxn], s2[maxn];
void print(char c1, char c2) {
  for (int i = 0; i < 26; ++i) {
    if ('a' + i != c1 && 'a' + i != c2) {
      putchar('a' + i);
      return;
    }
  }
}
int main() {
  int n, t;
  scanf("%d %d", &n, &t);
  scanf("%s %s", s1, s2);
  int cnt = 0;
  for (int i = 0; i < n; ++i)
    if (s1[i] != s2[i]) ++cnt;
  t = n - t;
  if ((cnt / 2 + (n - cnt)) < t)
    puts("-1");
  else {
    int rem = cnt / 2;
    rem = min(rem, t);
    t -= rem;
    rem *= 2;
    for (int i = 0; i < n; ++i) {
      if ((s1[i] != s2[i]) && rem) {
        if (rem & 1)
          putchar(s1[i]);
        else
          putchar(s2[i]);
        --rem;
        continue;
      }
      if ((s1[i] == s2[i]) && t) {
        putchar(s1[i]);
        --t;
        continue;
      }
      print(s1[i], s2[i]);
    }
    puts("");
  }
  return 0;
}