7810: [CSES Problem Set] Polygon Area
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Description
Your task is to calculate the area of a given polygon.
The polygon consists of $n$ vertices $(x_1,y_1),(x_2,y_2),…,(x_n,y_n)$. The vertices $(x_i,y_i)$ and $(x_{i+1},y_{i+1})$ are adjacent for $i=1,2,…,n−1$, and the vertices $(x_1,y_1)$ and $(x_n,y_n)$ are also adjacent.
The polygon consists of $n$ vertices $(x_1,y_1),(x_2,y_2),…,(x_n,y_n)$. The vertices $(x_i,y_i)$ and $(x_{i+1},y_{i+1})$ are adjacent for $i=1,2,…,n−1$, 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 $i$-th 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.
After this, there are $n$ lines that describe the vertices. The $i$-th 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
$3≤n≤1000$
$-10^9≤x_i,y_i≤10^9$
$-10^9≤x_i,y_i≤10^9$
Sample 1 Input
4
1 1
4 2
3 5
1 4
Sample 1 Output
16