#include <bits/stdc++.h>
using namespace std;
int32_t main() {
  long long n, k;
  string s1, s2, s3;
  long long c1 = 0, c2 = 0, diff = 0;
  cin >> n >> k;
  cin >> s1 >> s2;
  for (long long i = 0; i < n; i++)
    if (s1[i] != s2[i]) diff++;
  if ((diff + 1) / 2 > k) {
    cout << "-1\n";
    return 0;
  }
  diff = 0, s3 = s1;
  for (long long i = 0; i < n; i++) {
    for (char j = 'a'; j <= 'd'; j++) {
      if (s1[i] != s2[i] && s1[i] != j && s2[i] != j) {
        s3[i] = j;
        diff++;
        c1++;
        c2++;
        break;
      }
    }
  }
  for (long long i = 0; i < n && diff < k; i++) {
    for (char j = 'a'; j <= 'd'; j++) {
      if (s1[i] == s2[i] && s1[i] != j) {
        s3[i] = j;
        c1++;
        c2++;
        diff++;
        break;
      }
    }
  }
  for (long long i = 0; i < n && (c1 > k || c2 > k); i++) {
    for (char j = 'a'; j <= 'd'; j++) {
      if (s1[i] != s2[i] && s2[i] != j) {
        if (c1 > c2) {
          s3[i] = s1[i];
          c1--;
        } else {
          s3[i] = s2[i];
          c2--;
        }
        break;
      }
    }
  }
  if (c1 == k && c2 == k)
    cout << s3;
  else
    cout << "-1";
}
