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Problems

The area of ​​a polygon

The area of ​​a polygon

Time limit 1 second
Memory limit 64 MiB

Determine the area of the polygon.

Input data

The first line of one number - N (3 ≤ N ≤ 100000). Next N lines in a pair of numbers - the coordinates of another vertex simple polygon in order to circumvent or counterclockwise. All coordinates are integers modulo not exceeding 10000.

Output data

One number - the area of the reduced polygon with an accuracy of 1 decimal places.

Examples

Input example #1
3
1 0
0 1
1 1
Output example #1
0.5