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;
using ll = long long;
using ull = unsigned long long;
using vi = vector<int>;
using ii = pair<int, int>;
using vii = vector<pair<int, int>>;
char different(char a, char b) {
  for (char i = 'a'; i <= 'z'; ++i)
    if (i != a && i != b) return i;
  return 'a';
}
void solve() {
  int n, k;
  cin >> n >> k;
  string a, b;
  cin >> a >> b;
  int t = 0, li = 0;
  string c = b;
  for (int i = 0; i < n; ++i) {
    if (a[i] == b[i]) continue;
    if (t & 1) {
      c[i] = a[i];
    } else {
      c[i] = b[i];
      li = i;
    }
    ++t;
  }
  if (t & 1) {
    c[li] = different(a[li], b[li]);
    t++;
  }
  k *= 2;
  if (t > k) {
    cout << "-1\n";
    return;
  }
  int i = 0;
  while (t < k && i < n) {
    if (a[i] == b[i]) {
      c[i] = different(a[i], b[i]);
      t += 2;
    }
    ++i;
  }
  i = 0;
  while (t < k && i < n) {
    if (a[i] != b[i]) {
      c[i] = different(a[i], b[i]);
      ++t;
    }
    ++i;
  }
  if (t != k) {
    cout << "-1\n";
  } else
    cout << c << '\n';
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  int tc = 1;
  while (tc--) solve();
}