Polygon Area

Your task is to calculate the area of a given polygon. The polygon consists of n vertices (x_1,y_1),(x_2,y_2),\dots,(x_n,y_n). The vertices (x_i,y_i) and (x_{i+1},y_{i+1}) are adjacent for i=1,2,\dots,n-1, and the vertices (x_1,y_1) and (x_n,y_n) are also adjacent.

Input

The first input line has an integer n: the number of vertices. After this, there are n lines that describe the vertices. The ith such line has two integers x_i and y_i. You may assume that the polygon is simple, i.e., it does not intersect itself.

Output

Print one integer: 2a where the area of the polygon is a (this ensures that the result is an integer).

Constraints

$3 \le n \le 1000$

$-10^9 \le x_i, y_i \le 10^9$

Example

Input

4
1 1
4 2
3 5
1 4

Output

16