#include <bits/stdc++.h>
using namespace std;
char s[100010];
int main() {
  int n, t;
  cin >> n >> t;
  string s1, s2;
  cin >> s1 >> s2;
  for (int i = 0; i < n; ++i) {
    for (char j = 'a'; j <= 'z'; ++j) {
      if (s1[i] != j and s2[i] != j) {
        s[i] = j;
        break;
      }
    }
  }
  int same2;
  int same1 = same2 = n - t;
  for (int i = 0; i < n; ++i) {
    if (!same1) break;
    if (s1[i] == s2[i]) {
      s[i] = s1[i];
      same1--;
      same2--;
    }
  }
  for (int i = 0; i < n; ++i) {
    if (!same2) break;
    if (s1[i] != s2[i]) {
      if (same1 >= same2) {
        same1--;
        s[i] = s1[i];
      } else {
        same2--;
        s[i] = s2[i];
      }
    }
  }
  if (same1 or same2)
    cout << -1;
  else
    cout << s;
  return 0;
}
