a, b = map(int, input().split())
c, d = map(int, input().split())
ans = 11111111111111111111111111
for k1 in range(0, 1100):
for k2 in range(0, 1100):
if (d - c) + d * k2 == (b - a) + b * k1:
ans = min(ans, (d - c) + d * k2)
print(ans)