#include <bits/stdc++.h>
using namespace std;
int n, k, len;
string s1, s2;
char diff(char c1, char c2) {
  for (char c = 'a'; c <= 'z'; c++)
    if (c != c1 && c != c2) return c;
}
int main() {
  cin >> n >> k;
  getline(cin, s1, '\n');
  getline(cin, s1, '\n');
  getline(cin, s2, '\n');
  for (int i = 0; i <= n - 1; i++) len += (s1[i] != s2[i]);
  if (len == k) {
    for (int i = 0; i <= n - 1; i++)
      if (s1[i] == s2[i])
        cout << s1[i];
      else
        cout << diff(s1[i], s2[i]);
  } else if (len < k) {
    int dem = len;
    for (int i = 0; i <= n - 1; i++)
      if (s1[i] != s2[i])
        cout << diff(s1[i], s2[i]);
      else if (dem < k) {
        dem++;
        cout << diff(s1[i], s2[i]);
      } else
        cout << s1[i];
  } else {
    if (2 * k < len) {
      cout << -1;
      return 0;
    }
    int dem1 = 0, dem2 = 0;
    for (int i = 0; i <= n - 1; i++)
      if (s1[i] == s2[i])
        cout << s1[i];
      else if (dem1 < len - k) {
        cout << s1[i];
        dem1++;
      } else if (dem2 < len - k) {
        cout << s2[i];
        dem2++;
      } else
        cout << diff(s1[i], s2[i]);
  }
  return 0;
}
