#include <iostream>
using namespace std;
typedef long long LL;
int main(int argc, char* argv[])
{

	LL A,B,C;
	cin>>A>>B>>C;
	double X=(double)3600*C/(double)(A*60-B);
	if (X<0){
		cout<<-1<<endl;
		return 0;
	}
	if (X==(LL)X){
		cout<<X<<endl;
	}else{
		cout<<(LL)X+1<<endl;
	}

	return 0;
}