#include <cstdio>
#include <cstring>
#include <iostream>
#include <string>
#include <cmath>
#include <bitset>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <algorithm>
#include <complex>
#include <unordered_map>
using namespace std;
typedef long long int ll;
typedef pair<int, int> P;

int main()
{
	ll a, b, c;
	cin>>a>>b>>c;
	a*=60;
	c*=3600;
	if(a<=b){
		cout<<-1<<endl;
	}else{
		if(c%(a-b)==0) cout<<c/(a-b)<<endl;
		else cout<<c/(a-b)+1<<endl;
	}
	return 0;
}