Language: C++

#include<iostream>
#include<cfloat>
#include<cassert>
#include<cmath>
#include<vector>

using namespace std;

#define EPS (1e-8)
#define equals(a, b) (fabs((a) - (b)) < EPS )
#define dle(a, b) (equals(a, b) || a < b )

static const double PI = acos(-1);

class Point{
    public:
    double x, y;
    
    Point ( double x = 0, double y = 0): x(x), y(y){}
    
    Point operator + ( Point p ){ return Point(x + p.x, y + p.y); }
    Point operator - ( Point p ){ return Point(x - p.x, y - p.y); }
    Point operator * ( double a ){ return Point(x*a, y*a); }
    Point operator / ( double a ){ return Point(x/a, y/a); }

    double abs() { return sqrt(norm());}
    double norm() { return x*x + y*y; }

    bool operator < ( const Point &p ) const {
	return x != p.x ? x < p.x : y < p.y;
    }

    bool operator == ( const Point &p ) const {
	return fabs(x-p.x) < EPS && fabs(y-p.y) < EPS;
    }
};

typedef Point Vector;

class Segment{
    public:
    Point p1, p2;
    Segment(Point s = Point(), Point t = Point()): p1(s), p2(t){}
};

typedef Segment Line;

typedef vector<Point> Polygon;

double norm( Vector a ){ return a.x*a.x + a.y*a.y; }
double abs( Vector a ){ return sqrt(norm(a)); }
Point polar( double a, double r ){ return Point(cos(r)*a, sin(r)*a);}
double getDistance( Vector a, Vector b ){ return abs(a - b); }
double dot( Vector a, Vector b ){ return a.x*b.x + a.y*b.y; }
double cross( Vector a, Vector b ){ return a.x*b.y - a.y*b.x; }

static const int COUNTER_CLOCKWISE = 1;
static const int CLOCKWISE = -1;
static const int ONLINE_BACK = 2;
static const int ONLINE_FRONT = -2;
static const int ON_SEGMENT = 0;

// EPS can be 0
// need to check for 920, 833, 866
int ccw( Point p0, Point p1, Point p2 ){
    Vector a = p1 - p0;
    Vector b = p2 - p0;
    if ( cross(a, b) > EPS ) return COUNTER_CLOCKWISE;
    if ( cross(a, b) < -EPS ) return CLOCKWISE;
    if ( dot(a, b) < -EPS ) return ONLINE_BACK;
    if ( norm(a) < norm(b) ) return ONLINE_FRONT;
    return ON_SEGMENT;
}

// intersect Segment p1-p2 and Segment p3-p4 ?
bool isIntersect(Point p1, Point p2, Point p3, Point p4){
    return ( ccw(p1, p2, p3) * ccw(p1, p2, p4) <= 0 &&
	     ccw(p3, p4, p1) * ccw(p3, p4, p2) <= 0 );
}

// intersect Segment s1 and Segment s2 ?
// verified by 920, 833, 866, uoa2062
bool isIntersect(Segment s1, Segment s2){
    return isIntersect(s1.p1, s1.p2, s2.p1, s2.p2);
}


Polygon andrewScan( Polygon s ){
    Polygon u, l;
    if ( s.size() < 3 ) return s;
    sort(s.begin(), s.end());
    u.push_back(s[0]);
    u.push_back(s[1]);
    l.push_back(s[s.size()-1]);
    l.push_back(s[s.size()-2]);
    int n;
    for ( int i = 2; i < s.size(); i++ ){
	for ( int n = u.size(); n >= 2 && ccw(u[n-2], u[n-1], s[i]) != CLOCKWISE; n--){
	    u.pop_back();
	}
	u.push_back(s[i]);
    }

    for ( int i = s.size()-3; i >= 0; i-- ){
	for ( int n = l.size(); n >= 2 && ccw(l[n-2], l[n-1], s[i]) != CLOCKWISE; n--){
	    l.pop_back();
	}
	l.push_back(s[i]);
    }
    reverse(l.begin(), l.end());
    for ( int i = u.size()-2; i >= 1; i-- ) l.push_back(u[i]);
    return l;
}

main(){
  int n;
  char ch;
  double x, y;
  while( cin >> n && n ){
    Polygon P, C;
    for ( int i = 0; i < n; i++ ){
      cin >> x >>ch>> y;
      P.push_back(Point(x, y));
    }

    C = andrewScan(P);

    cout << P.size() - C.size() << endl;
  }
  
}

Compiler output:
a.cc: In function 'Polygon andrewScan(Polygon)':
a.cc:91:5: error: 'sort' was not declared in this scope; did you mean 'sqrt'?
   91 |     sort(s.begin(), s.end());
      |     ^~~~
      |     sqrt
a.cc:110:5: error: 'reverse' was not declared in this scope
  110 |     reverse(l.begin(), l.end());
      |     ^~~~~~~
a.cc: At global scope:
a.cc:115:1: warning: ISO C++ forbids declaration of 'main' with no type [-Wreturn-type]
  115 | main(){
      | ^~~~