typedef pair<int,int> par;
#define x first
#define y second

int ccw( par A, par B, par C ) {
  B.x -= A.x; B.y -= A.y;
  C.x -= A.x; C.y -= A.y;
  return B.x*C.y - C.x*B.y;
}

vector<par> convex_hull( vector<par> V ) {
  sort( V.begin(), V.end() );
  vector<par> A;
  for( int i = 0; i < V.size(); ++i ) {
    while( A.size() >= 2 && ccw( A[A.size()-2], A[A.size()-1], V[i] ) < 0 ) A.pop_back();
    A.push_back( V[i] );
  }
  for( int i = V.size()-2; i >= 0; --i ) {
    while( A.size() >= 2 && ccw( A[A.size()-2], A[A.size()-1], V[i] ) < 0 ) A.pop_back();
    A.push_back( V[i] );
  }
  A.pop_back();
  return A;
}