fn main() {
    input! {
        a: u64,
        b: u64,
        c: u64,
        d: u64,
    }
    let mut a = a;
    let mut b = b;
    let mut c = c;
    let mut d = d;
    if a * d < b * c {
        std::mem::swap(&mut a, &mut c);
        std::mem::swap(&mut b, &mut d);
    }
    let g = gcd(a, b);
    a /= g;
    b /= g;
    let g = gcd(c, d);
    c /= g;
    d /= g;
    if (a, b) == (c, d) {
        println!("-1");
        return;
    }
    // (a/b - c/d)x >= 1
    // (ad - bc) / bd * x >= 1
    let x = b * d / (a * d - b * c);
    let ans = x - (floor_sum(x, 2 * b, 2 * a, 2 * a + b) - floor_sum(x, 2 * d, 2 * c, 2 * c + d));
    println!("{}", ans);
}

fn gcd(a: u64, b: u64) -> u64 {
    if b == 0 {
        a
    } else {
        gcd(b, a % b)
    }
}

// ---------- begin input macro ----------
// reference: https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8
#[macro_export]
macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
    ($($r:tt)*) => {
        let s = {
            use std::io::Read;
            let mut s = String::new();
            std::io::stdin().read_to_string(&mut s).unwrap();
            s
        };
        let mut iter = s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
}

#[macro_export]
macro_rules! input_inner {
    ($iter:expr) => {};
    ($iter:expr, ) => {};
    ($iter:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($iter, $t);
        input_inner!{$iter $($r)*}
    };
}

#[macro_export]
macro_rules! read_value {
    ($iter:expr, ( $($t:tt),* )) => {
        ( $(read_value!($iter, $t)),* )
    };
    ($iter:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>()
    };
    ($iter:expr, chars) => {
        read_value!($iter, String).chars().collect::<Vec<char>>()
    };
    ($iter:expr, bytes) => {
        read_value!($iter, String).bytes().collect::<Vec<u8>>()
    };
    ($iter:expr, usize1) => {
        read_value!($iter, usize) - 1
    };
    ($iter:expr, $t:ty) => {
        $iter.next().unwrap().parse::<$t>().expect("Parse error")
    };
}
// ---------- end input macro ----------
// ---------- begin floor sum ----------
// sum_{i = 0}^{n - 1} floor((ai + b) / m)
pub fn floor_sum(n: u64, m: u64, mut a: u64, mut b: u64) -> u64 {
    let mut ans = 0;
    ans += b / m * n;
    if n % 2 == 0 {
        ans += a / m * (n / 2) * (n - 1);
    } else {
        ans += a / m * n * ((n - 1) / 2);
    }
    a %= m;
    b %= m;
    let p = a * n + b;
    if p >= m {
        ans += floor_sum(p / m, a, m, p % m);
    }
    ans
}
// ---------- end floor sum ----------