//凸包，Graham演算法
//點編號0~n-1
//返回凸包結果Stack[0~Top-1]
Point List[maxn],Stack[maxn]; int Top;
bool cmp( Point a , Point b )
{
    double tmp = (a-List[0])^(b-List[0]);
    if ( tmp>0 ) return true;
    else if ( tmp==0&&sgn( dist2( a , List[0] )-dist2( b , List[0] ) )<=0 ) return true;
    else return false;
}
void Graham( int n )
{
    int k = 0;
    for ( int i=1 ; i<n ; i++ )
        if ( List[i].y<List[k].y||(List[i].y==List[k].y&&List[i].x<List[k].x) ) k = i;
    swap ( List[k] , List[0] );
    sort ( List+1  , List+n , cmp );
    if ( n==1 )
    {
        Top = 1;
        Stack[0] = List[0];
        return;
    }
    if ( n==2 )
    {
        Top = 2;
        Stack[0] = List[0];
        Stack[1] = List[1];
        return;
    }
    Stack[0] = List[0];
    Stack[1] = List[1];
    Top = 2;
    for ( int i=2 ; i<n ; i++ )
    {
        while ( Top>1&&((Stack[Top-1]-Stack[Top-2])^(List[i]-Stack[Top-2]))<=0 ) Top--;
        Stack[Top++] = List[i];
    }
}