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;
long long int gcd(long long int a, long long int b) {
  if (b == 0) return a;
  return (a % b == 0 ? b : gcd(b, a % b));
}
template <typename P>
struct Cmp {
  bool operator()(const P &p1, const P &p2) {
    if (p1.first > p2.first) return true;
    if (p1.first == p2.first) return p1.second < p2.second;
    return false;
  }
};
int main() {
  int n, t;
  scanf("%d", &n), scanf("%d", &t);
  string s1, s2;
  cin >> s1 >> s2;
  int z = n - t;
  char s[s1.length() + 1];
  for (int i = int(0); i < int(s1.length() + 1); i++) {
    s[i] = '*';
  }
  map<int, int> m1;
  for (int i = 0; i < s1.length(); i++) {
    if (s1[i] == s2[i] && z > 0) {
      s[i] = s1[i];
      m1[i] = 1;
      z--;
    } else {
      for (int j = int(97); j < int(97 + 26); j++) {
        char ch = (char)j;
        if (ch == s1[i] || ch == s2[i]) {
          continue;
        } else {
          s[i] = ch;
          break;
        }
      }
    }
  }
  if (!z) {
    s[s1.length()] = '\0';
    cout << s << endl;
    return 0;
  }
  int ct1 = z, ct2 = z;
  for (int i = int(0); i < int(s1.length()); i++) {
    if (m1[i]) continue;
    if (ct1 > 0) {
      s[i] = s1[i];
      ct1--;
    } else if (ct2 > 0) {
      s[i] = s2[i];
      ct2--;
    }
    if (ct1 == 0 && ct2 == 0) {
      break;
    }
  }
  if (ct1 == 0 && ct2 == 0) {
    s[s1.length()] = '\0';
    cout << s << endl;
  } else {
    puts("-1");
  }
  return 0;
}