use std::io::{stdin, Read};

fn main() {
    let mut buf = String::new();
    stdin().read_to_string(&mut buf).unwrap();
    let mut tok = buf.split_whitespace();
    let mut get = || tok.next().unwrap();
    macro_rules! get {
        ($t:ty) => (get().parse::<$t>().unwrap());
        () => (get!(u64));
    }
    
    let a = get!() * 60;
    let b = get!();
    let c = get!() * 60 * 60;
    
    // a*x < b*x + c (better manual)
    // x < c/(a-b)
    // x < ceil(c/(a-b))
    // a*x >= b*x + c (better auto)
    // (a-b)*x >= c
    // x >= c/(a-b)
    // x > floor(c/(a-b))
    
    let max = 1_000_000_000_000_000; 
    
    let mut lp = 0;
    let mut rp = max;
    while lp < rp {
        let x = (lp + rp) / 2;
        let ax = a * x;
        let bxc = b * x + c;
        if ax <= bxc {
            lp = x + 1;
        } else {
            rp = x;
        }
    }
    if rp < max {
        println!("{}", rp);
    } else {
        println!("-1");
    }
}