#include<iostream>
#include<cassert>
using namespace std;
int main()
{
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	char x,y;
	int n;cin>>x>>y>>n;
	if(n==1)
	{
		if(x==y)cout<<x<<"\n";
		else cout<<"-1\n";
	}
	else
	{
		for(int i=0;i<n-1;i++)cout<<x;
		cout<<y<<"\n";
	}
}