#include <bits/stdc++.h>
using namespace std;

using i32 = int;
using i64 = long long;
using i128 = __int128_t;
using f64 = double;
using p2 = pair<i64, i64>;
using p3 = tuple<i64, i64, i64>;

void _main();
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(18);
  _main();
}

void _main() {
  char s, e;
  i32 n;
  cin >> s >> e >> n;
  if (n == 1) {
    if (s != e) cout << "-1\n";
    else cout << s << "\n";
  } else {
    cout << s << string(n - 2, 'a') << e << "\n";
  }
}