实现计算多边形面积的功能。第一行输入多边形的顶点数。第二行以后按照次序依次输入坐标点（如“2 3”,代表X 坐标和Y 坐标，中间加入空格区分）-To achieve the function of calculating the polygon area. Enter the first line of the polygon vertices. After the second line followed in the order of the input coordinates of points (such as " 2 3" , on behalf of X coordinates and Y coordinates, the distinction between the middle add a space)