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;
int main() {
  int taille, nbDiffs;
  cin >> taille >> nbDiffs;
  int nbMemes = taille - nbDiffs;
  string a, b;
  cin >> a >> b;
  string c = a;
  vector<int> posMemes, posDiffs;
  for (int pos = 0; pos < taille; pos++) {
    if (a[pos] == b[pos])
      posMemes.push_back(pos);
    else
      posDiffs.push_back(pos);
  }
  for (int pos : posMemes) {
    if (nbMemes == 0) {
      c[pos] = ((c[pos] + 1 - 'a') % 26) + 'a';
    } else {
      nbMemes--;
    }
  }
  for (int fois = 0; fois < nbMemes; fois++) {
    if (posDiffs.empty()) {
      cout << "-1" << endl;
      return 0;
    }
    c[posDiffs.back()] = a[posDiffs.back()];
    posDiffs.pop_back();
  }
  for (int fois = 0; fois < nbMemes; fois++) {
    if (posDiffs.empty()) {
      cout << "-1" << endl;
      return 0;
    }
    c[posDiffs.back()] = b[posDiffs.back()];
    posDiffs.pop_back();
  }
  for (int pos : posDiffs) {
    char ac = a[pos];
    char bc = b[pos];
    for (char car = 'a'; car <= 'z'; car++) {
      if (car != ac && car != bc) {
        c[pos] = car;
      }
    }
  }
  cout << c << endl;
  return 0;
}