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) {
  return b ? gcd(b, a % b) : a;
}
long long int lcm(long long int a, long long int b) {
  return a * b / gcd(a, b);
}
int n, k;
string s, t;
set<int> st;
char a[101010];
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0), cout.tie(0);
  cin >> n >> k >> s >> t;
  if (k > n) return cout << -1 << endl, 0;
  for (char i = 'a'; i <= 'z'; i++) st.insert(i);
  memset(a, '1', sizeof(a));
  a[n] = '\0';
  int ne = n - k, b = n;
  for (int i = 0; i < n; i++)
    if (s[i] == t[i]) {
      if (ne == 0) break;
      a[i] = s[i];
      ne--;
      b--;
    }
  if (ne * 2 > b) return cout << -1 << endl, 0;
  int ns = ne, nt = ne;
  for (int i = 0; i < n; i++)
    if (!isalpha(a[i]) && s[i] != t[i]) {
      if (!ns) break;
      a[i] = s[i];
      ns--;
    }
  for (int i = 0; i < n; i++) {
    if (!isalpha(a[i]) && s[i] != t[i]) {
      if (!nt) break;
      a[i] = t[i];
      nt--;
    }
  }
  for (int i = 0; i < n; i++) {
    if (isalpha(a[i])) continue;
    st.erase(s[i]);
    st.erase(t[i]);
    a[i] = *(st.begin());
    st.insert(s[i]);
    st.insert(t[i]);
  }
  cout << a << endl;
  return 0;
}