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;
int n, t, a, b, tp;
string s1, s2;
char s3[100010];
vector<int> posa, posb;
char distinto(char a, char b) {
  for (char x = 'a'; x <= 'z'; x++)
    if (x != a && x != b) return x;
  return '%';
}
int main() {
  ios::sync_with_stdio(false);
  while (cin >> n >> t) {
    cin >> s1 >> s2;
    for (int i = (0); i < (n); i++) s3[i] = '$';
    s3[n] = 0;
    tp = n - t;
    a = b = 0;
    for (int i = (0); i < (n); i++)
      if (s1[i] == s2[i]) {
        posa.push_back(i);
        a++;
      } else {
        posb.push_back(i);
        b++;
      }
    if (tp <= a || (tp > a && 2 * (tp - a) <= b)) {
      int c = min(a, tp);
      for (int i = (0); i < (c); i++) s3[posa[i]] = s1[posa[i]];
      if (tp > a) {
        int c = (tp - a);
        for (int i = (0); i < (c); i++) {
          s3[posb[i]] = s1[posb[i]];
        }
        for (int i = (c); i < (2 * c); i++) {
          s3[posb[i]] = s2[posb[i]];
        }
      }
      for (int i = (0); i < (n); i++)
        if (s3[i] == '$') s3[i] = distinto(s1[i], s2[i]);
      cout << s3 << endl;
    } else {
      cout << -1 << endl;
    }
    posa.clear();
    posb.clear();
  }
  return 0;
}