fn main() {
    let stdin = std::io::read_to_string(std::io::stdin()).unwrap();
    let mut stdin = stdin.split_ascii_whitespace();

    let a: u32 = stdin.next().unwrap().parse().unwrap();
    let b: u32 = stdin.next().unwrap().parse().unwrap();
    let c: u32 = stdin.next().unwrap().parse().unwrap();

    println!("{}", output(solve(a, b, c)));
}

fn solve(a: u32, b: u32, c: u32) -> Option<u64> {
    match a as u64 * 60 <= b as u64 {
        true => None,
        false => {
            let cost_cut = a as u64 * 60 - b as u64;
            Some((c as u64 * 3600 + cost_cut - 1) / cost_cut)
        }
    }
}

fn output(ans: Option<u64>) -> String {
    match ans {
        Some(ans) => ans.to_string(),
        None => String::from("-1"),
    }
}