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

int main() {
	char s, e;
	cin >> s >> e;
	int N; 
	cin >> N;

	if (N == 1)
	{
		if (s == e)
			cout << s << "\n";
		else
			cout << -1 << "\n";
		return 0;
	}

	string answer = string(N, 'k');
	answer[0] = s;
	answer[N - 1] = e;
	cout << answer << "\n";
	return 0;
}