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 xi, yi (-1000 ≤ xi, yi ≤ 1000) are given.

Output

Print the area of the polygon with 3 decimal digits.