- Time limit: 1.00 s
- Memory limit: 512 MB
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