double sum = 0;
 int m = 1;
 for(;;)
 {
  if(NumPoint < 3)break;
  FPoint r1;
  r1.x = pArray[m].x - pArray[0].x;
  r1.y = pArray[m].y - pArray[0].y;
  FPoint r2;
  r2.x = pArray[m+1].x - pArray[m].x;
  r2.y = pArray[m+1].y - pArray[m].y;
  sum += (r1.x * r2.y - r2.x * r1.y);
  m++;
  NumPoint --;
 }
 double result = (sum>0)?(sum/2.):(-sum/2.);
 return result;
}
//test
void main()
{
 FPoint pArray1[4]={{1,1},{1,2},{2,2},{2,1}};
 FPoint pArray2[5]={{0,0},{0,2},{2,2},{1,1},{2,0}};
 FPoint pArray3[9]={{0,0},{2,0},{2,1},{0,1},{0,2},{-2,0},{-1,0},{-1,-1},{0,-1}};
 printf("Area1 = %f\n",MianJi(pArray1,4));
 printf("Area2 = %f\n",MianJi(pArray2,5));
 printf("Area3 = %f\n",MianJi(pArray3,9));
}