#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 i=1;
	while(1){
		if(i>ceil((3600*c)/(60*a-b))){
			cout << i << endl;
			return 0;
		}
		i++;
	}
	return 0;
}