Полезная площадь
"But I do write will not, I'll save to buy a cow" - responded to the offer Matroskin Pechkin subscribe to the magazine or newspaper.
E. Uspenskiy "Three of Prostokvashino"
Finally Burenka led Matroskin calf ... But after a couple of months when he had to detach a calf from the mother's self-grazing, the cat faced a new problem, because the calf always wanted to be near his mother.
Problem Matroskin decided quickly and radically, blocking the square corral, which had the shape of a parallelogram on one of the diagonals: one half of the grazing Burenka, and the second - the calf. However, the as woozy, ran to his paddock and then the ball advised his friend to tie the calf rope.
Here is where the problem arose, which Matroskin itself can not solve, and asks for your help: help him to tie a calf at such a point of his pen to graze in the area that was the highest.
Input
In one line, four integers: a, b, c - the length of the fence corral a calf and d - the length of the rope to which it is bound (0 < a, b, c, d ≤ 100). All numbers are separated by a space.
Output
Single number - a useful area of pasture bred to an accuracy of 2 decimal places.