e-olymp
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 (3n50000) lines the integer coordinates of consecutive vertices xi, yi (-1000xi, yi1000) are given.

Output

Print the area of the polygon with 3 decimal digits.

Time limit 1 second
Memory limit 128 MiB
Input example #1
3
0 0
0 2
2 0
Output example #1
2.000