#include <bits/stdc++.h>
using namespace std;
char s3[100005];
int main() {
  int n, t;
  cin >> n >> t;
  string s1;
  string s2;
  cin >> s1 >> s2;
  int req = n - t;
  for (int i = 0; i < n; i++) {
    if (req == 0) break;
    if (s1[i] == s2[i]) {
      s3[i] = s1[i];
      req--;
    }
  }
  if (req != 0) {
    int temp = req;
    for (int i = 0; i < n; i++) {
      if (!temp && req && s3[i] != s1[i]) {
        s3[i] = s2[i];
        req--;
      }
      if (temp && s3[i] != s1[i]) {
        s3[i] = s1[i];
        temp--;
      }
    }
  }
  if (req) {
    puts("-1");
    return 0;
  }
  for (int i = 0; i < n; i++) {
    if (s3[i] != s1[i] && s3[i] != s2[i]) {
      for (char c = 'a'; c < 'z'; c++)
        if (c != s1[i] && c != s2[i]) {
          s3[i] = c;
          break;
        }
    }
  }
  for (int i = 0; i < n; i++) cout << s3[i];
}
