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 MAX_N = 100005;
char s1[MAX_N], s2[MAX_N], s3[MAX_N];
int flag[MAX_N];
int main() {
  int n, t;
  scanf("%d %d\n", &n, &t);
  scanf("%s", &s1);
  scanf("%s", &s2);
  int cnt = 0;
  int x = n - t;
  for (int i = 0; i < n; i++) {
    if (cnt == x) break;
    if (s1[i] == s2[i]) {
      cnt++;
      s3[i] = s1[i];
      flag[i] = 1;
    }
  }
  int cnt1 = cnt;
  for (int i = 0; i < n; i++)
    if (flag[i] == 0) {
      if (cnt1 == x) break;
      s3[i] = s1[i];
      flag[i] = 1;
      cnt1++;
    }
  int cnt2 = cnt;
  for (int i = n - 1; i >= 0; i--)
    if (flag[i] == 0) {
      if (cnt2 == x) break;
      s3[i] = s2[i];
      flag[i] = 1;
      cnt2++;
    }
  if (cnt2 < x) {
    cout << -1;
    return 0;
  }
  for (int i = 0; i < n; i++)
    if (flag[i] == 1)
      printf("%c", s3[i]);
    else {
      for (char ch = 'a'; ch <= 'c'; ch++)
        if (s1[i] != ch && s2[i] != ch) {
          printf("%c", ch);
          break;
        }
    }
  return 0;
}