#include <bits/stdc++.h>
using namespace std;
long long n, t;
string a, b, c, d;
void doc() {
  cin >> n >> t;
  cin >> a;
  a = " " + a;
  cin >> b;
  b = " " + b;
}
char khac(long k) {
  if ('a' != a[k] and b[k] != 'a') return 'a';
  if ('b' != a[k] and b[k] != 'b') return 'b';
  if ('c' != a[k] and b[k] != 'c') return 'c';
}
void xl() {
  c = " ";
  for (int i = 1; i <= n; i++) c += khac(i);
  t = n - t;
  for (int i = 1; i <= n; i++) {
    if (a[i] == b[i] and t > 0) {
      c[i] = a[i];
      t--;
    }
  }
  long na = 0, nb = 0;
  for (int i = 1; i <= n; i++) {
    if (a[i] != c[i] and b[i] != c[i] and na < t) {
      c[i] = a[i];
      na++;
    }
    if (a[i] != c[i] and b[i] != c[i] and nb < t) {
      c[i] = b[i];
      nb++;
    }
  }
  if (na < t or nb < t) {
    cout << "-1";
    return;
  }
  for (int i = 1; i <= n; i++) d += c[i];
  cout << d;
}
int main() {
  cin.tie(NULL);
  ios_base::sync_with_stdio(false);
  doc();
  xl();
}
