Description

In front of the newbie HansBug is a peculiar magnetic field region in the shape of a polygon, described by a sequence of $N$ points, which are the vertices of the region in order. Now HansBug wants to know the magnetic flux of this region. However, his brain cells and luck have been depleted, so this high-end task is left to you.

Input Format

The first line contains an integer $N$ and a real number $B$, where $B$ is the magnetic flux density.

Lines $2\sim N+1$ each contain two real numbers, representing the $x$ and $y$ coordinates of a point.

Output Format

Output one line containing a real number: the magnetic flux (rounded to 4 decimal places).

3 1
0 0
1 0
0 2

1.0000

4 1
1 1
1 -1
-1 -1
-1 1

4.0000

Hint

Magnetic flux $=$ magnetic flux density $\times$ area of the magnetic field region.

$0\lt B\le 10$.

Translated by ChatGPT 5