X, Y, Z, W = map(int, input().split())
lb = 0
ub = X / Y - Z
while ub - lb > 0.000001:
    mid = (ub + lb) / 2
    a = X / (mid + Z) - Y
    tmp = ((mid + Z) ** 2 + Y ** 2) ** 0.5 - ((a + Y) ** 2 + Z ** 2) ** 0.5 - W
    if tmp > 0:
        ub = mid
    else:
        lb = mid

ans = X - ((ub + Z) * Y + (a + Y) * Z + a * ub) / 2
print(ans)