#include <bits/stdc++.h>
using namespace std;
int main() {
	char s, e;
	int n;
	cin >> s >> e >> n;
	if (n == 1 && s != e) {
		cout << -1 << endl;
	} else {
		cout << s + string(n - 1, e) << endl;
	}
	return 0;
}