#define _USE_MATH_DEFINES
#include<iostream>
#include<vector>
#include<algorithm>
#include<cmath>
#include<string>
#include<iomanip>
#include<numeric>
#include<queue>
#include<deque>
#include<stack>
#include<set>
#include<map>
#include<random>
using namespace std;
typedef long long ll;
const int mod=1e9+7;
int main(){
	ll a,b,c;
	cin>>a>>b>>c;
	cout<<(60*a<=b?-1:(3600*c+60*a-b-1)/(60*a-b))<<endl;
}