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

def is_ok(k):
    p = ((k+w)**2-y**2)*(k**2-z**2)
    return p >= x**2
ng = 0
ok = 2*x
while abs(ng-ok) > 10**(-9):
    mid = (ng+ok)/2
    if is_ok(mid):
        ok = mid
    else:
        ng = mid
k = ok
import math
a = math.sqrt((k+w)**2-y**2)
b = math.sqrt(k**2-z**2)
ans = x-y*a/2-b*z/2-(b-y)*(a-z)/2
print(ans)