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 HBase = 1E4 + 7;
int read() {
  int x;
  scanf("%d", &x);
  return x;
}
int n, t, p[100000];
string a, b, s;
char check(char ch1, char ch2) {
  if (ch1 != 'a' && ch2 != 'a') return 'a';
  if (ch1 != 'b' && ch2 != 'b') return 'b';
  return 'c';
}
int main() {
  ios_base::sync_with_stdio(false);
  cin >> n >> t;
  cin >> a;
  cin >> b;
  int l = 0, r = -1;
  s = "";
  s.resize(n);
  for (int i = 0, _sz = (n); i < _sz; ++i)
    if (a[i] != b[i]) {
      p[++r] = i;
      if (t > 0) {
        s[i] = check(a[i], b[i]);
        --t;
      } else {
        if (r - l + 1 >= 2) {
          s[p[l]] = a[p[l]];
          s[p[r]] = b[p[r]];
          ++l;
          --r;
        } else {
          cout << -1;
          return 0;
        }
      }
    } else
      s[i] = a[i];
  for (int i = 0, _sz = (n); i < _sz; ++i)
    if (t > 0 && a[i] == b[i]) {
      s[i] = check(a[i], b[i]);
      --t;
    }
  cout << s;
  return 0;
}