#include <bits/stdc++.h>
#define rep(i, a, n) for(int i = a; i < n; i++)
#define repr(i, a, b) for(int i = a; i >= b; i--)
#define int long long
#define all(a) a.begin(), a.end()
#define chmax(x, y) x = max(x, y)
#define chmin(x, y) x = min(x, y)
using namespace std;
typedef pair<int, int> P;
const int mod = 1000000007;
const int INF = 1e15;

signed main(){
	ios::sync_with_stdio(false);
	cin.tie(0);
	int a, b, c;
	cin >> a >> b >> c;
	if(60 * a - b <= 0){
		cout << -1 << endl;
		return 0;
	}
	int ans = (3600 * c + 60 * a - b - 1) / (60 * a - b);
	cout << ans << endl;
}