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 double PI = 2.0 * acos(0.0);
const int MOD = 1e9 + 7;
const int oo = MOD;
long long gcd(long long x, long long y) { return !y ? x : gcd(y, x % y); }
int main() {
  int n, k;
  cin >> n >> k;
  string a, b, ans;
  cin >> a >> b;
  int x, y, cnt = 0;
  x = y = k;
  ans = string(n, '!');
  for (int i = 0; i < n; i++) cnt += (a[i] != b[i]);
  if (cnt > k * 2) {
    puts("-1");
    return 0;
  }
  cnt = 2 * k - cnt;
  for (int i = 0; i < n; i++)
    if (a[i] != b[i] && cnt) {
      for (int j = 0; j < 26; j++)
        if (j + 'a' != a[i] && j + 'a' != b[i]) {
          ans[i] = j + 'a';
          --cnt, --x, --y;
          break;
        }
    }
  for (int i = 0; i < n; i++)
    if (ans[i] == '!' && a[i] != b[i]) {
      if (x > y)
        ans[i] = a[i], --x;
      else
        ans[i] = b[i], --y;
    }
  for (int i = 0; i < n; i++)
    if (ans[i] == '!' && a[i] == b[i]) {
      if (x) {
        if (a[i] == 'a')
          ans[i] = 'b';
        else
          ans[i] = 'a';
        --x, --y;
      } else
        ans[i] = a[i];
    }
  cout << ans << '\n';
  return 0;
}