#include <algorithm>
#include <iostream>
#include <vector>
#include <queue>
#include <set>
#include <map>
#include <list>
#include <cmath>
#include <stack>
#include <string>
#include <cstring>
using namespace std;
//
const int inf=1e9+7;//0x3f
//
int main(){
	long long A,B,C;
	cin>>A>>B>>C;
	if(60*A-B<=0){
		cout<<-1<<endl;
	}else{
		cout<<(3600*C+60*A-B-1)/(60*A-B)<<endl;
	}
	return 0;
}