I summarise all corners of this polygon and see: if a>0(360)->ccw;
else (a=-360)->cw; Why is it wrong?

This is my program:

#include<iostream.h>
#include<math.h>

double len(double x1,double y1,double x2,double y2)
{return sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));}

double gon(double x1,double y1,double x2,double y2,double x3,double
y3)
{return asin(((x2-x1)*(y3-y2)-(x3-x2)*(y2-y1))/(len(x1,y1,x2,y2)*len
(x2,y2,x3,y3)));}

int main()
{
    double a=0,x0,y0,x1,y1,xl,yl,xr,yr,xn,yn;
    long n,i; cin >> n;
    cin >> x0 >> y0 >> x1 >> y1;
    xl=x0; yl=y0; xr=x1; yr=y1;
    for (i=3; i<=n; i++)
    {
        cin >> xn >> yn;
        a+=gon(xl,yl,xr,yr,xn,yn);
        xl=xr; yl=yr; xr=xn; yr=yn;
    }
    a+=gon(xl,yl,xr,yr,x0,y0);
    a+=gon(xr,yr,x0,y0,x1,y1);
    if (a<0) cout << "cw"; else cout << "ccw";
    return 0;
}