#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, t, di;
  cin >> n >> t;
  string a, b;
  cin >> a >> b;
  string c;
  t = n - t;
  int cn = 0;
  for (int i = 0; i < n; i++) {
    if (a[i] == b[i]) cn++;
  }
  di = t - cn;
  if (n - cn < 2 * di) {
    cout << -1;
    return 0;
  }
  int ch = 0;
  for (int i = 0; i < n; i++) {
    if (a[i] == b[i]) {
      if (t) {
        c += a[i];
        t--;
      } else {
        char df = 'a';
        while (df == a[i] || df == b[i]) df++;
        c += df;
      }
    } else if (di > 0) {
      if (ch == 0)
        c += a[i];
      else {
        c += b[i];
        di--;
      }
      ch ^= 1;
    } else {
      char df = 'a';
      while (df == a[i] || df == b[i]) df++;
      c += df;
    }
  }
  cout << c << endl;
  return 0;
}
