fn main() {
    let mut b = String::new();
    std::io::stdin().read_line(&mut b).ok();
    let b: Vec<isize> = b.trim().split_whitespace().map(|s| s.parse().unwrap()).collect();
    let b1 = b[0];
    let b2 = b[1];
    let b3 = b[2];

    if b1 == b2 {
        println!("{}", b3);
    } else {
        println!("{}", (b3 - b2) * (b3 - b2) / (b2 - b1) + b3);
    }
}