#include <bits/stdc++.h>
using namespace std;
const long long mod = 1000000007;
long long powmod(long long a, long long b) {
  long long res = 1;
  a %= mod;
  for (; b; b >>= 1) {
    if (b & 1) res = res * a % mod;
    a = a * a % mod;
  }
  return res;
}
int main() {
  ios_base::sync_with_stdio(0);
  int n, t;
  cin >> n >> t;
  string s1, s2;
  cin >> s1 >> s2;
  int d = 0;
  int l = s1.length();
  for (int i = 0; i < l; i++)
    if (s1[i] != s2[i]) d++;
  if (d > t + t)
    cout << "-1\n";
  else {
    int p = t - d;
    int q = t + t - d;
    int f = 0;
    for (int i = 0; i < l; i++) {
      if (s1[i] == s2[i]) {
        if (p <= 0)
          cout << s1[i];
        else {
          cout << (s1[i] == 'a' ? 'b' : 'a');
          p--;
        }
      } else {
        if (q <= 0) {
          if (f) {
            cout << s1[i];
            f = 0;
          } else {
            cout << s2[i];
            f = 1;
          }
        } else {
          if (s1[i] != 'a' && s2[i] != 'a')
            cout << 'a';
          else if (s1[i] != 'b' && s2[i] != 'b')
            cout << 'b';
          else
            cout << 'c';
          q--;
        }
      }
    }
    cout << endl;
  }
  return 0;
}
