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 mod = 998244353;
long long int INF = 1000000000000000000;
long long int powmod(long long int a, long long int b) {
  long long int res = 1;
  a %= mod;
  assert(b >= 0);
  for (; b; b >>= 1) {
    if (b & 1) res = res * a % mod;
    a = a * a % mod;
  }
  return res;
}
long long int gcd(long long int a, long long int b) {
  return b ? gcd(b, a % b) : a;
}
const long long int MAXN = 300005;
void magic(int n, int *ar) {
  set<int> s;
  map<int, int> m;
  for (int i = 0; i < n; i++) s.insert(ar[i]);
  int size = 0;
  for (set<int>::iterator it = s.begin(); it != s.end(); it++) {
    m[*it] = size++;
  }
  for (int i = 0; i < n; i++) ar[i] = m[ar[i]];
}
bool present(string s, string r) {
  size_t found = s.find(r);
  if (found != string::npos) {
    return 1;
  }
  return 0;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int t = 1;
  while (t--) {
    long long int n, k, sum = 0, x = 1, y = 1, cnt = 0, res = 0, flag = 0, t;
    cin >> n >> t;
    string r, s;
    char ans[n];
    cin >> r >> s;
    long long int p = 0;
    t = n - t;
    for (int i = (0); i < (n); i++) {
      if (r[i] == s[i] && p < t) {
        p++;
        ans[i] = r[i];
      } else {
        if (r[i] != 'a' && s[i] != 'a') {
          ans[i] = 'a';
        } else if (r[i] != 'b' && s[i] != 'b') {
          ans[i] = 'b';
        } else {
          ans[i] = 'c';
        }
      }
    }
    long long int a = t - p, b = t - p;
    for (int i = (0); i < (n); i++) {
      if (r[i] != s[i]) {
        if (a) {
          a--;
          ans[i] = r[i];
        } else if (b) {
          b--;
          ans[i] = s[i];
        } else {
          if (r[i] != 'a' && s[i] != 'a') {
            ans[i] = 'a';
          } else if (r[i] != 'b' && s[i] != 'b') {
            ans[i] = 'b';
          } else {
            ans[i] = 'c';
          }
        }
      }
    }
    if (a || b) {
      cout << -1 << "\n";
      continue;
    }
    for (int i = (0); i < (n); i++) cout << ans[i];
    cout << "\n";
  }
  return 0;
}