Problems

# The Area of the Polygon

# The Area of the Polygon

The coordinates of **n** consecutive points of polygon are given. Find the area of the polygon.

#### Input

The number of polygon vertices **n** is given in the first line. In next **n** (**3** ≤ **n** ≤ **50000**) lines the integer coordinates of consecutive vertices `x`

, _{i}`y`

(_{i}**-1000** ≤ `x`

, _{i}`y`

≤ _{i}**1000**) are given.

#### Output

Print the area of the polygon with **3** decimal digits.

Input example #1

3 0 0 0 2 2 0

Output example #1

2.000