#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
#define pb push_back
#define mp make_pair
#define rep(i,n) for(int i=0;i<(n);++i)

int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);
	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;
	}
}