#include <iostream>
#include <string>
#include <algorithm>
#include <vector>
#include <math.h>
#include <iomanip>

using namespace std;

int main(){
	long long a,b,c;
	cin >> a >> b >> c;

	if(60*a<b){
		cout << -1 << endl;
		return 0;
	}
	long long ans=(3600*c)/(60*a-b)+1;
	cout << ans<< endl;
	return 0;
}