Polygon Area (Shoelace)

From AOJ. Shoelace formula for polygon area. Solve the geometry problem "Polygon Area (Shoelace)".

Examples
Input: [1,2,3]
Output: 0
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Polygon Area (Shoelace)

**From AOJ.** Shoelace formula for polygon area. Solve the geometry problem "Polygon Area (Shoelace)".