#include<iostream>
#include<cassert>
using namespace std;
int main()
{
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	long A,B,C;
	cin>>A>>B>>C;
	//A*60*X>=X*B+C*3600
	//(60*A-B)*X>=C*3600
	long x=60*A-B,y=C*3600;
	if(x<=0)cout<<-1<<endl;
	else cout<<(y+x-1)/x<<endl;
}