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 long long mod = 1000000007;
long long powmod(long long a, long long b) {
  long long res = 1;
  a %= mod;
  for (; b; b >>= 1) {
    if (b & 1) res = res * a % mod;
    a = a * a % mod;
  }
  return res;
}
int main() {
  ios_base::sync_with_stdio(0);
  int n, t;
  cin >> n >> t;
  string s1, s2;
  cin >> s1 >> s2;
  int d = 0;
  int l = s1.length();
  for (int i = 0; i < l; i++)
    if (s1[i] != s2[i]) d++;
  if (d > t + t)
    cout << "-1\n";
  else {
    int p = t - d;
    int q = t + t - d;
    int f = 0;
    for (int i = 0; i < l; i++) {
      if (s1[i] == s2[i]) {
        if (p <= 0)
          cout << s1[i];
        else {
          cout << (s1[i] == 'a' ? 'b' : 'a');
          p--;
        }
      } else {
        if (q <= 0) {
          if (f) {
            cout << s1[i];
            f = 0;
          } else {
            cout << s2[i];
            f = 1;
          }
        } else {
          if (s1[i] != 'a' && s2[i] != 'a')
            cout << 'a';
          else if (s1[i] != 'b' && s2[i] != 'b')
            cout << 'b';
          else
            cout << 'c';
          q--;
        }
      }
    }
    cout << endl;
  }
  return 0;
}