Useful Area#1100
A farmer had a cow that recently gave birth to a calf. After a few months, when it was time to separate the calf from its mother for independent grazing, the farmer faced a new problem: the calf always wanted to be near its mother.
The farmer quickly solved the problem by dividing the corral, which had the shape of a parallelogram, along one of the diagonals: in one half grazed the cow, and in the other grazed the calf. However, the restless calf kept running around in its area, so the farmer's friend suggested tying the calf with a rope.
Now the farmer faces a problem he cannot solve alone and asks for your help: help him tie the calf at such a point in its pen so that the grazing area is maximized.
On one line, four integers: , , — the lengths of the fence of the calf's corral, and — the length of the rope (0 < a, b, c, d \leq 100). All numbers are separated by spaces.
A single number — the useful grazing area, printed with an accuracy of decimal places.
Example 1