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

#define rep(i, a, b) for (int i = a; i < b; i++)
using ll = long long;

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 cout << string(n - 1, 's') + string(1, e) << endl;
}