#include <bits/stdc++.h>
long long int mod = 1000000007;
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  ;
  int n, t;
  cin >> n >> t;
  string s1, s2;
  cin >> s1;
  cin >> s2;
  int a = 0, b = 0;
  int x = (n - t);
  for (int i = 0; i < n; i++) {
    if (s1[i] == s2[i]) {
      a++;
    } else {
      b++;
    }
  }
  string ans = "";
  if (x <= a) {
    for (int i = 0; i < n; i++) {
      if (s1[i] == s2[i] && x > 0) {
        ans += s1[i];
        x--;
      } else {
        for (char c = 'a'; c <= 'z'; c++) {
          if (c != s1[i] && c != s2[i]) {
            ans += c;
            break;
          }
        }
      }
    }
    cout << ans << endl;
    return 0;
  }
  if (2 * (x - a) > (n - a)) {
    cout << -1 << endl;
    return 0;
  }
  int cnt = 0;
  for (int i = 0; i < n; i++) {
    if (s1[i] == s2[i]) {
      ans += s1[i];
    } else {
      if (cnt < (x - a)) {
        ans += s1[i];
        cnt++;
      } else if (cnt < (2 * (x - a))) {
        ans += s2[i];
        cnt++;
      } else {
        for (char c = 'a'; c <= 'z'; c++) {
          if (c != s1[i] && c != s2[i]) {
            ans += c;
            break;
          }
        }
      }
    }
  }
  cout << ans << endl;
  return 0;
}
