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 int mod = 1e9 + 7;
const long long int INF = 9e18 + 2e17;
const int inf = 2e9;
const int N = 1e3 + 22;
const double eps = 1e-10;
const double PI = 3.14159265;
char get(char a, char b) {
  if (a == b) {
    if (a == 'z')
      return a - 1;
    else
      return a + 1;
  }
  if (a > b) swap(a, b);
  if (b == a + 1) {
    if (b == 'z')
      return a - 1;
    else
      return b + 1;
  }
  return a + 1;
}
void solve() {
  int n, t;
  cin >> n >> t;
  string a, b;
  cin >> a >> b;
  int same = 0, diff = 0;
  for (int i = 0; i < n; i++) {
    if (a[i] == b[i])
      same++;
    else
      diff++;
  }
  if ((diff + 1) / 2 > t) {
    cout << -1;
    return;
  }
  int same_dec = 0;
  if (t > diff) same_dec = t - diff;
  int diff_dec = 0;
  if (diff > t) diff_dec = diff - t;
  int p = 0;
  for (int i = 0; i < n; i++) {
    if (a[i] == b[i]) {
      if (same_dec) {
        cout << get(a[i], b[i]);
        same_dec--;
      } else
        cout << a[i];
    } else {
      if (diff_dec) {
        if (p == 0) {
          cout << a[i];
          p = 1;
        } else {
          cout << b[i];
          p = 0;
          diff_dec--;
        }
      } else
        cout << get(a[i], b[i]);
    }
  }
  cout << '\n';
  return;
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  solve();
  return 0;
}