The area of a polygon

Input

The first line contains one integer $N$ (3 \leq N \leq 100000). Each of the next $N$ lines contains a pair of integers, which are the coordinates of each vertex of a simple polygon given in either clockwise or counterclockwise order. All coordinates are integers with absolute values not exceeding $10000$.

Output

Output a single number: the area of the given polygon, accurate to $1$ decimal place.

Examples