Problem Statement

In this problem, $\ |AB|\ $ denotes the length of the segment $\ AB\ $ that has points $\ A\ $ and $\ B\ $ as its endpoints and connects these $\ 2\ $ points.

On a two-dimensional plane, there is a rectangle $\ ABCD\ $, with a point $\ P\ $ on side $\ AD\ $ and a point $\ Q\ $ on side $\ CD\ $.
They also satisfy the following.

• The area of rectangle $\ ABCD\ $ is $\ X\ $
• Points $\ P\ $ and $\ Q\ $ are not at the same position
• $|BP|=|BQ|+W$
• $|AP|=Y$
• $|CQ|=Z$

Find the area of triangle $\ BPQ\ $.

Input

$X\ \ Y\ \ Z\ \ W$

• $1 \leq Y \times Z < X \leq 10^6$
• $1 \leq Y,Z$
• $0 \leq W \leq 10^6$
• All inputs are integers
• There exist a rectangle $\ ABCD,\ $ a point $\ P,\ $ and a point $\ Q\ $ satisfying the conditions

Output

Output the area of triangle $\ BPQ\ $, followed by a newline.
Your answer is considered correct if its relative or absolute error from the expected solution is at most $\ 10^{-5}\ $.

Sample

96 5 6 3

33.0000000000

16 2 2 0

6.0000000000