fn main() {
	let mut s = String::new();
	std::io::stdin().read_line(&mut s).ok();
	let n: Vec<i64> = s.split_whitespace().flat_map(str::parse).collect();
	let d = n[0] * 60 - n[1];
	if d <= 0 {
		println!("-1");
		return;
	}
	println!("{}", (n[2] * 3600 + d - 1) / d)
}