X, Y, Z, W = map(int, input().split())
l, r = 0, 10 ** 6 + 1
eps = 10 ** -6
while r - l > eps:
    m = (r + l) / 2
    if (m ** 2 + Y ** 2) ** .5 - ((X / m) ** 2 + Z ** 2) ** .5 > W:
        r = m
    else:
        l = m
AB, AC = X / l, l
ans = X - AB * Y / 2 - AC * Z / 2 - (AC - Y) * (AB - Z) / 2
print(ans)