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 int d[8][2] = {{-1, -1}, {-1, 0}, {-1, 1}, {0, -1},
                     {0, 1},   {1, -1}, {1, 0},  {1, 1}};
const int N = 1e5 + 10;
const int mod = 1e9 + 7;
using namespace std;
string a;
string b;
string c;
int n, k, l;
char ret(char v, char u) {
  for (char t = 'a'; t <= 'z'; t++) {
    if (t != v && t != u) return t;
  }
  return 'a';
}
int main() {
  cin >> n >> k;
  cin >> a >> b;
  for (int i = 0; i < n; i++) {
    c += '.';
    if (a[i] == b[i]) l++;
  }
  k = n - k;
  if ((n - l) / 2 + l < k) {
    cout << -1;
    return 0;
  }
  int u = k - min(k, l);
  int v = k - min(k, l);
  for (int i = 0; i < n; i++) {
    if (a[i] == b[i] && k != 0) {
      c[i] = a[i];
      k--;
    } else if (u >= v && u != 0) {
      u--;
      c[i] = a[i];
    } else if (v > u && v != 0) {
      v--;
      c[i] = b[i];
    } else
      c[i] = ret(a[i], b[i]);
  }
  cout << c;
}