#include <bits/stdc++.h>
using namespace std;
const long long mod = 1000000007;
char find_char(char a) {
  for (char j = 'a'; j <= 'z'; j++) {
    if (a != j) return j;
  }
}
char find_char(char a, char b) {
  for (char j = 'a'; j <= 'z'; j++) {
    if (a != j && b != j) return j;
  }
}
int main(int argc, char const *argv[]) {
  int n, t;
  while (cin >> n >> t) {
    int cnt = 0;
    string a;
    string b;
    cin >> a >> b;
    for (int i = 0; i < n; i++) {
      if (a[i] == b[i]) cnt++;
    }
    int _t = n - t;
    if (_t > cnt + (n - cnt) / 2) {
      printf("-1\n");
    } else {
      if (_t > cnt) {
        int temp1 = (_t - cnt);
        int temp2 = temp1;
        int ret = 1;
        for (int i = 0; i < n; i++) {
          if (a[i] == b[i]) {
            cout << a[i];
          } else {
            if (ret == 1) {
              cout << a[i];
              temp1--;
              if (temp1 == 0) {
                ret = 2;
              }
            } else if (ret == 2) {
              cout << b[i];
              temp2--;
              if (temp2 == 0) {
                ret = 3;
              }
            } else {
              printf("%c", find_char(a[i], b[i]));
            }
          }
        }
        printf("\n");
      } else {
        int temp = cnt - _t;
        for (int i = 0; i < n; i++) {
          if (a[i] != b[i]) {
            printf("%c", find_char(a[i], b[i]));
          } else {
            if (temp != 0) {
              temp--;
              printf("%c", find_char(a[i]));
            } else {
              cout << a[i];
            }
          }
        }
        printf("\n");
      }
    }
  }
  return 0;
}
