#include<bits/stdc++.h>

using namespace std;

int main()
{
	char s, e;
	int n;
	cin >> s >> e >> n;
	if (n == 1) {
		if (s == e) cout << s << endl;
		else cout << -1 << endl;
	} else {
		string ans(n, 'a');
		ans[0] = s;
		ans[n - 1] = e;
		cout << ans << endl;
	}
}