x, y, z, w = map(int, input().split())

ok_ad = y
ng_ad = x / z
while abs(ok_ad - ng_ad) > 1e-9:
    ad = (ok_ad + ng_ad) / 2
    cd = x / ad
    bp = (y**2 + cd**2) ** 0.5
    bq = (z**2 + ad**2) ** 0.5

    if bp > bq + w:
        ok_ad = ad
    else:
        ng_ad = ad

ad = ok_ad
cd = x / ad

bpq = x - (y * cd + z * ad + (ad-y) * (cd - z)) / 2
print(bpq)