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

int main() {
	char a, b;
	int S;
	cin >> a >> b >> S;
	if (S == 1 && a != b) {cout << -1 << endl;  return 0;}
	if (S == 1) {cout << a << endl;  return 0;}
	else {
		cout << a;
		for (int i = 2; i < S; i++) cout << 'a';
		cout << b;
		return 0;
	}
}