import math

def extgcd(a, b):
    if b:
        d, y, x = extgcd(b, a % b)
        y -= (a // b) * x
        return d, x, y
    return a, 1, 0

def remainder(V):
    x = 0; d = 1
    for X, Y in V:
        g, a, b = extgcd(d, Y)
        x, d = (Y*b*x + d*a*X) // g, d*(Y // g)
        x %= d
    return x, d

def main():
    # Aで割るとa余り，Bで割るとb余る数のうち，最小のものを求める
    A,B,a,b = map(int, input().split())
    assert 1<=A<=3000
    assert 1<=B<=3000
    assert 0<=a<A
    assert 0<=b<B
    assert math.gcd(A,B)==1
    print(remainder([(a,A), (b,B)])[0])
    
if __name__ == "__main__":
    main()