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 poi