#include <bits/stdc++.h>
using namespace std;
char m(char a, char b);
char c[100000];
int main() {
  int n, t;
  cin >> n >> t;
  string a, b;
  cin >> a >> b;
  int count, counter(-1);
  count = n - t;
  for (int i = 0; i < n; i++) {
    if (count == 0)
      c[i] = m(a[i], b[i]);
    else {
      if (a[i] == b[i]) {
        c[i] = a[i];
        count--;
      } else if (a[i] != b[i] && counter == -1) {
        counter = i;
        c[i] = m(a[i], b[i]);
      } else if (a[i] != b[i] && counter != -1) {
        c[i] = b[i];
        c[counter] = a[counter];
        counter = -1;
        count--;
      }
    }
  }
  if (count == 0) {
    puts(c);
  } else
    cout << "-1";
}
char m(char a, char b) {
  if (a == 'a' || b == 'a') {
    if (a == 'b' || b == 'b') {
      return 'c';
    } else
      return 'b';
  } else
    return 'a';
}
