fn main() {
    let mut abc = String::new();
    std::io::stdin().read_line(&mut abc).ok();
    let abc: Vec<usize> = abc.trim().split_whitespace().map(|s| s.parse().unwrap()).collect();
    let a = abc[0] * 60;
    let b = abc[1];
    let c = abc[2] * 3600;

    if a <= b {
        println!("-1");
    } else {
        println!("{}", c / (a-b) + if c % (a-b) > 0 { 1 } else { 0 });
    }
}