point and polygon

#include <iostream>
#include <stdio.h>
#include <cmath>

using namespace std;

struct pt {
    int x,y;
} a[111111],p;

int n;

int det ( pt a, pt b, pt c )
{
    return (b.x - a.x) * (c.y - b.y) - (c.x - b.x ) * (b.y - a.y );
}
bool intersect( pt a,pt b,pt c,pt d )
{
    return det(a,b,c) * det(a,b,d) <= 0 && det(c,d,a) * det(c,d,b) < 0;
}
int main()
{
    scanf("%d",&n);
    for ( int i = 1; i <= n; i++ )
        scanf("%d%d",&a[i].x,&a[i].y);
    scanf("%d%d",&p.x,&p.y);

    if ( det(a[1],a[n],p) > 0 || det(a[1],a[2],p) < 0 ) { puts("NO"); return 0; }
    int l = 2, r = n;
    while ( r - l > 1 ) {
        int m = (r + l) / 2;
        if ( det(a[1],a[m],p) >= 0 ) l = m;
            else r = m;

    }
    if ( intersect(a[1],p,a[l],a[r]) ) puts("NO");
        else puts("YES");
}