#include <iostream>
#include <cmath>

using namespace std;
int gcd(long long , long long);

int main()
{
    long long a, b, c, d, k, ans = 0, m;
    cin >> a >> b >> c >> d;
    while (a * d < c * b){
        a++;
        b++;
        m = gcd(a, b);
        a /= m;
        b /= m;
        ans++;
        //cout << m << endl;
    }
    if (a == c && b == d)
        cout << ans << endl;
    else
        cout << 0 << endl;
    return 0;
}

int gcd (long long a, long long b) {
	if (b == 0)
		return a;
	else
		return gcd (b, a % b);
}

/*int gcd(long long a, long long b){
    long m, n, z;
    m = a;
    n = b;
    while (n != 0){
        z = m % n;
        m = n;
        n = z;
    }
    return m;
}*/