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>
#pragma comment(linker, "/STACK:1024000000,1024000000")
using namespace std;
const int INF = 0x3f3f3f3f;
const long long INFLL = 0x3f3f3f3f3f3f3f3f;
const int MAXN = 1e5 + 5;
const int MAXM = MAXN;
const int mod = 1e9 + 7;
char s[MAXN], t[MAXN], p[MAXN];
char getdiff(char a, char b) {
  for (int i = 0; i < 26; i++)
    if (a != i + 'a' && b != i + 'a') return i + 'a';
}
int main() {
  int n, m, diff = 0;
  cin >> n >> m >> s >> t;
  for (int i = 0; i < n; i++)
    if (s[i] != t[i]) diff++;
  if ((diff + 1) / 2 > m)
    puts("-1");
  else {
    int v = 1;
    int same = n - diff;
    for (int i = 0; i < n; i++) {
      if (s[i] == t[i]) {
        if ((diff + 1) / 2 < m) {
          p[i] = getdiff(s[i], t[i]);
          m--;
        } else
          p[i] = s[i];
        same--;
      } else {
        if (diff / 2 + same >= m && diff > 1) {
          if (v & 1)
            p[i] = s[i];
          else
            p[i] = t[i], m--;
          if (v % 2 == 0) diff -= 2;
          v++;
        } else {
          p[i] = getdiff(s[i], t[i]);
          diff--;
          m--;
        }
      }
    }
    if (m)
      puts("-1");
    else {
      p[n] = '\0';
      cout << p << endl;
    }
  }
  return 0;
}