#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  int n, t;
  cin >> n >> t;
  string s1, s2;
  string s3 = "";
  cin >> s1 >> s2;
  for (int i = 1; i <= n; i++) {
    s3 += '?';
  }
  int diff1 = n - t, diff2 = n - t;
  for (int i = 0; i < n; i++) {
    if (diff1 > 0 && diff2 > 0 && s1[i] == s2[i]) {
      s3[i] = s1[i];
      diff1--;
      diff2--;
    }
  }
  for (int i = 0; i < n; i++) {
    if (diff1 == 0) break;
    if (s3[i] == '?' && s1[i] != s2[i]) {
      s3[i] = s1[i];
      diff1--;
    }
  }
  for (int i = 0; i < n; i++) {
    if (diff2 == 0) break;
    if (s3[i] == '?' && s2[i] != s1[i]) {
      s3[i] = s2[i];
      diff2--;
    }
  }
  if (!(diff1 == 0 && diff2 == 0)) {
    cout << "-1\n";
    exit(0);
  }
  assert(diff1 == 0 && diff2 == 0);
  for (int i = 0; i < n; i++) {
    if (s3[i] != '?') continue;
    for (int j = 0; j < 26; j++) {
      char ch = (char)'a' + (char)j;
      if (ch != s1[i] && ch != s2[i]) {
        s3[i] = ch;
        break;
      }
    }
  }
  cout << s3 << "\n";
}
