fn main() {
	let mut s = String::new();
	std::io::stdin().read_line(&mut s).ok();
	let n: Vec<u64> = s.split_whitespace().flat_map(str::parse).collect();
	if n[0] * 60 <= n[1] {
		println!("-1");
		return;
	}
	let mut x = 0;
	while n[0] * 60 * x < n[1] * x + n[2] * 3600 {
		x += 1;
	}
	println!("{}", x)
}