#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long int n, t;
  cin >> n >> t;
  string s1, s2;
  cin >> s1 >> s2;
  string ans(n, 0);
  long long int tmp = n - t;
  for (long long int i = 0; i < n; i++) {
    if (tmp == 0) break;
    if (s1[i] == s2[i]) {
      tmp--;
      ans[i] = s1[i];
    }
  }
  long long int f = tmp, s = tmp;
  for (long long int i = 0; i < n; i++) {
    if (f == 0 and s == 0) break;
    if (f > 0 and s1[i] != s2[i]) {
      ans[i] = s1[i];
      f--;
    } else if (s > 0 and s1[i] != s2[i]) {
      ans[i] = s2[i];
      s--;
    }
  }
  for (long long int i = 0; i < n; i++) {
    if (ans[i] == 0) {
      if (s1[i] != 'a' and s2[i] != 'a')
        ans[i] = 'a';
      else if (s1[i] != 'b' and s2[i] != 'b')
        ans[i] = 'b';
      else
        ans[i] = 'c';
    }
  }
  if (f > 0 or s > 0)
    cout << -1;
  else
    cout << ans;
  return 0;
}
