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

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	char s, e;
	int n;
	cin >> s >> e >> n;
	string ans(n, s);
	ans.back() = e;
	cout << (ans[0] != s ? "-1" : ans) << '\n';
}