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:256000000")
using namespace std;
const double infd = 2e+9;
const int infi = (1 << 30) - 1;
const long long infl = (1ll << 60) - 1;
template <class T>
inline T sqr(T x) {
  return x * x;
}
char getDiff(char a, char b) {
  char c = 'a';
  if (c == a || c == b) c++;
  if (c == a || c == b) c++;
  if (c == a || c == b) c++;
  if (c == a || c == b) c++;
  return c;
}
int main() {
  int n, t;
  cin >> n >> t;
  string a, b;
  cin >> a >> b;
  string c = a;
  for (int i = 0; i < n; i++) c[i] = '!';
  int eqCount = 0, notEqCount = 0;
  for (int i = 0; i < n; i++) {
    if (a[i] == b[i]) {
      eqCount++;
    } else
      notEqCount++;
  }
  if (!(notEqCount - notEqCount / 2 <= t && t <= notEqCount + eqCount)) {
    cout << -1;
    return 0;
  }
  int t1 = 0, t2 = 0;
  for (int i = 0; i < n; i++) {
    if (a[i] != b[i]) {
      if (t1 >= t2)
        c[i] = a[i], t2++;
      else
        c[i] = b[i], t1++;
    }
  }
  for (int i = 0; i < n; i++) {
    if (a[i] != b[i]) {
      if (c[i] == a[i] && t1 < t) {
        c[i] = getDiff(a[i], b[i]);
        t1++;
      }
      if (c[i] == b[i] && t2 < t) {
        c[i] = getDiff(a[i], b[i]);
        t2++;
      }
    }
  }
  for (int i = 0; i < n; i++) {
    if (a[i] == b[i]) {
      if (t1 < t && t2 < t) {
        c[i] = getDiff(a[i], b[i]);
        t1++, t2++;
      } else
        c[i] = a[i];
    }
  }
  cout << c << endl;
  return 0;
}