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>
const double pi = acos(-1.0);
const long long mod = 1e9 + 7;
using namespace std;
char getDiff(char a, char b) {
  for (char res = 'a'; res <= 'z'; res++) {
    if (res != a && res != b) return res;
  }
}
char s1[100005], s2[100005], s3[100005];
int a[100005];
int main() {
  memset(a, 0, sizeof(a));
  int n, m;
  scanf("%d %d", &n, &m);
  scanf("%s %s", s1, s2);
  int diff = 0, same = 0;
  for (int i = 0; i < n; i++) {
    if (s1[i] != s2[i])
      diff++;
    else
      same++;
  }
  for (int i = 0; i < n; i++) {
    if (s1[i] == s2[i]) {
      if (m > diff) {
        m--;
        s3[i] = (s1[i] - 'a' + 1) % 26 + 'a';
      } else
        s3[i] = s1[i];
    }
  }
  if (m * 2 < diff) {
    puts("-1");
    return 0;
  }
  int single1 = diff - m, single2 = diff - m;
  for (int i = 0; i < n; i++) {
    if (s1[i] != s2[i]) {
      if (single1 != 0) {
        s3[i] = s1[i];
        single1--;
      } else if (single2 != 0) {
        s3[i] = s2[i];
        single2--;
      } else
        s3[i] = getDiff(s1[i], s2[i]);
    }
  }
  puts(s3);
  return 0;
}