#include<iostream>
#include<algorithm>
#include <vector>
#include<cmath>
#include<iomanip>
#include<queue>
using namespace std;
typedef long long int lont;
int main() {
	double A, B, C;
	cin >> A >> B >> C;
	A *= 60;
	C *= 3600;
	lont ans = 1;
	if (A <= B) {
		cout << -1 << endl;
	}
	else if(C/(A-B)-lont(C/(A-B))){
		cout << lont(C / (A - B))+1 << endl;
	}
	else {
		cout << lont(C / (A - B)) << endl;
	}
}