AriaPoligonSimplu

#include <cstdio>
using namespace std;
FILE *f=fopen("ariapoligonsimplu.in","r");
FILE *g=fopen("ariapoligonsimplu.out","w");
typedef struct
{
   int x,y;
} Point;

double PolygonArea(Point *polygon,int N)
{
   int i,j;
   double area = 0;

   for(i=0;i<N;i++)
   {
      j=(i+1)%N;
      area+=polygon[i].x*polygon[j].y;
      area-=polygon[i].y*polygon[j].x;
   }

   area/=2;
   return(area<0?-area:area);
}

int main()
{
    Point coord[100];
    int nr;
    fscanf(f,"%d",&nr);
    for (int i=0;i<nr;i++)
        fscanf(f,"%d%d",&coord[i].x,&coord[i].y);
    fprintf(g,"%1.1f",PolygonArea(coord,nr));
    return 0;
}