#include <bits/stdc++.h>
using namespace std;
void File() {
  freopen("in.txt", "r", stdin);
  freopen("out.txt", "w", stdout);
}
int main() {
  int n, t;
  cin >> n >> t;
  int match = n - t, mis = t;
  string s1, s2;
  cin >> s1 >> s2;
  int cnt = 0;
  for (int i = 0; i < n; ++i)
    if (s1[i] == s2[i]) cnt++;
  if (cnt >= match) {
    cnt = match;
    for (int i = 0; i < n; ++i) {
      if (cnt && s1[i] == s2[i]) {
        cout << s1[i];
        cnt--;
      } else {
        for (char j = 'a'; j <= 'z'; ++j) {
          if (j != s1[i] && j != s2[i]) {
            cout << j;
            break;
          }
        }
      }
    }
  } else if (cnt < match) {
    if ((match - cnt) * 2 + cnt > n) {
      puts("-1");
      return 0;
    }
    match -= cnt;
    match *= 2;
    bool bol = 0;
    for (int i = 0; i < n; ++i) {
      if (cnt && s1[i] == s2[i]) {
        cout << s1[i];
        cnt--;
      } else {
        if (match) {
          if (bol == 0) {
            cout << s1[i];
            bol = 1;
          } else {
            cout << s2[i];
            bol = 0;
          }
          match--;
        } else {
          for (char j = 'a'; j <= 'z'; ++j) {
            if (j != s1[i] && j != s2[i]) {
              cout << j;
              break;
            }
          }
        }
      }
    }
  }
  return 0;
}
