x, y, z, w = map(int, input().split())
l, r = 0, 10 ** 6
for i in range(200000):
    m = (l + r) / 2
    tate = x / m
    if tate < z:
        r = m
        continue
    BP = (tate ** 2 + y ** 2) ** 0.5
    BQ = (m ** 2 + z ** 2) ** 0.5
    if BQ + w > BP:
        r = m
    else:
        l = m
print(x - (r * z / 2 + (x / r) * y / 2 + (r - y) * (x / r - z) / 2))