def floor_sum_unsigned(n, m, a, b):
    ans = 0
    while True:
        if a >= m:
            ans += n * (n - 1) * (a // m) // 2
            a %= m
        if b >= m:
            ans += n * (b // m)
            b %= m

        y_max = a * n + b        
        if y_max < m: break
        n, b, m, a, = y_max // m, y_max % m, a, m
    return ans

A,B,C,D=map(int,input().split())
if (B*C==A*D):
    print(-1)
    exit()

if (B*C>A*D):
    A,C=C,A
    B,D=D,B

det=(A*D-B*C)
M=(B*D)//det
an=M-1
an-=floor_sum_unsigned(M,2*B,2*A,B)
an+=floor_sum_unsigned(M,2*D,2*C,D)
    
print(an)