the set of points in the convex hull
vector < pair < int , int > > a ; a . push_back ( { 0 , 0 } ) ; a . push_back ( { 3 , -1 } ) ; a . push_back ( { 4 , 5 } ) ; a . push_back ( { -1 , 4 } ) ; int n = a . size ( ) ; pair < int , int > p = { 100 , 100 } ; addPoint ( a , p ) ;