/* -*- coding: utf-8 -*-
 *
 * 3487.cc:  No.3487 Restricted Shiritori - yukicoder
 */

#include<cstdio>
#include<algorithm>

using namespace std;

/* main */

int main() {
  char s[4], t[4];
  int n;
  scanf("%s%s%d", s, t, &n);

  if (n == 1) {
    if (s[0] == t[0]) puts(s);
    else puts("-1");
  }
  else {
    for (int i = 1; i < n; i++) putchar(s[0]);
    putchar(t[0]);
    putchar('\n');
  }

  return 0;
}