Problem Statement

MtSaka is currently in a territorial dispute. In it, he managed to occupy $N$ points.
He will rule the interior of the $N$-gon formed by connecting those points.
Given the coordinates of all $N$ points, find $2$ times the area he rules.
Note that the answer is always an integer. It is also guaranteed that the polygon formed by connecting the points in the order given in the input is a convex polygon, and that the points are given in counterclockwise order.

Input

$N$
$X_1\ Y_1$
$X_2\ Y_2$
$\vdots$
$X_N\ Y_N$

Constraints

・$3 \leq N \leq 30$
・$-10000 \leq X_i,Y_i \leq 10000 (1 \leq i \leq N)$
・All input values are integers
・The input satisfies the conditions in the problem statement

Output

Output $2$ times the area of the $N$-gon ruled by MtSaka, as a single positive integer on one line. Print a newline at the end.

Sample

3
1 0
1 1
0 1

1

7
-1 -1
2 -1
2 3
1 5
0 6
-1 5
-2 3

42

6
4 1
9 1
11 4
9 9
7 8
5 5

72