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 GCC optimize("O2")
using namespace std;
const int INF = 1 << 29;
const int MOD = 1e9 + 7;
bool isPowerOfTwo(long long int x) { return x && (!(x & (x - 1))); }
void fastio() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
}
long long binpow(long long a, long long b) {
  long long res = 1;
  while (b > 0) {
    if (b & 1) res = res * a;
    a = a * a;
    b >>= 1;
  }
  return res;
}
const int dx[] = {1, 0, -1, 0, 1, 1, -1, -1};
const int dy[] = {0, -1, 0, 1, 1, -1, -1, 1};
int main() {
  fastio();
  int tc = 1;
  while (tc--) {
    int n, t;
    cin >> n >> t;
    string a, b;
    cin >> a >> b;
    int diff = 0;
    for (int i = 0; i < (n); i++) {
      if (a[i] != b[i]) diff++;
    }
    string ans = "";
    if (diff > 2 * t)
      cout << -1;
    else {
      int sm = t - diff;
      if (diff <= t) {
        for (int i = 0; i < (n); i++) {
          if (a[i] == b[i]) {
            if (!sm)
              ans += a[i];
            else {
              for (char j = 'a'; j <= 'z'; j++) {
                if (j != a[i]) {
                  ans += j;
                  break;
                }
              }
              sm--;
            }
          } else {
            for (char j = 'a'; j < 'z'; j++) {
              if (j != a[i] && j != b[i]) {
                ans += j;
                break;
              }
            }
            diff--;
          }
        }
      } else {
        int x = 2 * t - diff;
        int y1 = t - x, y2 = y1;
        for (int i = 0; i < (n); i++) {
          if (a[i] == b[i]) {
            ans += a[i];
            continue;
          } else {
            if (x) {
              for (char j = 'a'; j <= 'z'; j++) {
                if (j != a[i] && j != b[i]) {
                  ans += j;
                  x--;
                  break;
                }
              }
            } else {
              if (!y1) {
                swap(y1, y2);
                swap(a, b);
              }
              if (y1) {
                for (char j = 'a'; j <= 'z'; j++) {
                  if (j == a[i]) {
                    ans += j;
                    y1--;
                    break;
                  }
                }
              }
            }
          }
        }
      }
    }
    cout << ans;
  }
  return 0;
}