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

# V = [(X_i, Y_i), ...]: X_i (mod Y_i)
def remainder(V):
    x = 0; d = 1
    for X, Y in V:
        g, a, b = extgcd(d, Y)
        if x%g!=X%g:
            return 0,0
        x, d = (Y*b*x + d*a*X) // g, d*(Y // g)
        x %= d
    return x, d

A,B,a,b=map(int,input().split())
x,d=remainder([(a,A),(b,B)])
print(x)