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

using ll = int64_t;
using ul = uint64_t;
using vi = vector<int>;
using vd = vector<double>;
using vc = vector<char>;
using vs = vector<string>;
using vb = vector<bool>;
using vl = vector<ll>;
using vul = vector<ul>;
using pii = pair<int,int>;
using pll = pair<ll,ll>;
using sei = set<int>;
using sec = set<char>;
using sel = set<ll>;
using mii = map<int,int>;
using vvi = vector<vi>;
using vvd = vector<vd>;
using vvc = vector<vc>;
using vvb = vector<vb>;
using vvl = vector<vl>;

int main() {
  char s,e;
  int N;
  cin >> s >> e >> N;
  if (s != e && N == 1) {
    cout << -1 << endl;
  }
  else if (s == e && N == 1) {
    cout << s << endl;
  }
  else {
    for (int i = 0; i < N - 1; i++) {
      cout << s;
    }
    cout << e << endl;
  }
}