#include <iostream>
using namespace std;

typedef long long ll;

int main(void){
	ll a,b,c;
	cin>>a>>b>>c;
	a*=60LL;
	c*=3600LL;
	ll k=c/(a-b)+1LL;
	cout<<((k<0LL)?(-1):(k))<<endl;
	return 0;
}