#include <bits/stdc++.h>
using namespace std;
char ans[100005];
int main() {
  int n, k;
  cin >> n >> k;
  string s1, s2;
  cin >> s1 >> s2;
  k = n - k;
  for (int i = 0; i <= 100000; i++) {
    ans[i] = '!';
  }
  int sum1 = 0;
  int sum2 = 0;
  for (int i = 0; i < n; i++) {
    if (sum1 == k) {
      break;
    }
    if (s1[i] == s2[i]) {
      ans[i] = s1[i];
      sum1++;
      sum2++;
    }
  }
  int last = 2;
  for (int i = 0; i < n; i++) {
    if (sum1 == k && sum2 == k) break;
    if (ans[i] != '!') continue;
    if (last == 2 && sum1 < k) {
      ans[i] = s1[i];
      sum1++;
      if (sum2 < k) last = 1;
    } else if (last == 1 && sum2 < k) {
      ans[i] = s2[i];
      sum2++;
      if (sum1 < k) last = 2;
    }
  }
  if (sum1 < k || sum2 < k) {
    cout << -1;
    return 0;
  }
  for (int i = 0; i < n; i++) {
    if (ans[i] != '!') continue;
    for (char j = 'a'; j <= 'z'; j++) {
      if (j != s1[i] && j != s2[i]) {
        ans[i] = j;
        break;
      }
    }
  }
  for (int i = 0; i < n; i++) cout << ans[i];
}
