Language: C

#include <stdio.h>
#include <string.h>
#include <math.h>

struct POINT{
	double x,y;
};

double area(double a,double b,double c){
	double s;

	s = (a+b+c)/2.0;

	return sqrt(s*(s-a)*(s-b)*(s-c));
}

int main(){
	int h,i,j,k,l,n,cnt;
	int flag[100];
	double len[6],s[4];
	POINT p[4];
	POINT pin[100];

	while(1){
		scanf("%d",&n);
		if(!n){
			break;
		}
		for(i = 0;i < n;i++){
			scanf("%lf,%lf",&pin[i].x,&pin[i].y);
		}

		memset(flag,0,sizeof(flag));

		for(i = 0;i < n;i++){
			if(flag[i]){
				continue;
			}
			p[0] = pin[i];
			for(j = 0;j < n;j++){
				if(i == j || flag[j]){
					continue;
				}
				p[1] = pin[j];
				for(k = 0;k < n;k++){
					if(i == k || j == k || flag[k]){
						continue;
					}
					p[2] = pin[k];
					for(h = 0;h < 3;h++){
							len[h] = sqrt(pow(p[(h+1)%3].x-p[h].x,2)+pow(p[(h+1)%3].y-p[h].y,2));
					}
					s[3] = area(len[0],len[1],len[2]);
					for(l = 0;l < n;l++){
						if(i == l || j == l || k == l || flag[l]){
							continue;
						}
						p[3] = pin[l];
						for(h = 0;h < 3;h++){
							len[3+h] = sqrt(pow(p[3].x-p[h].x,2)+pow(p[3].y-p[h].y,2));
						}
						for(h = 0;h < 3;h++){
							s[h] = area(len[h],len[3+h],len[(h+1)%3+3]);
						}
						if(fabs(s[3]-s[0]-s[1]-s[2])/(s[0]+s[1]+s[2]) < 1e-5){
							flag[l] = 1;
						}
					}
				}
			}
		}

		cnt = 0;
		for(i = 0;i < n;i++){
			if(flag[i]){
				cnt++;
			}
		}

		printf("%d\n",cnt);
	}

	return 0;
}

Compiler output:
main.c: In function 'main':
main.c:21:9: error: unknown type name 'POINT'; use 'struct' keyword to refer to the type
   21 |         POINT p[4];
      |         ^~~~~
      |         struct 
main.c:22:9: error: unknown type name 'POINT'; use 'struct' keyword to refer to the type
   22 |         POINT pin[100];
      |         ^~~~~
      |         struct 
main.c:30:48: error: request for member 'x' in something not a structure or union
   30 |                         scanf("%lf,%lf",&pin[i].x,&pin[i].y);
      |                                                ^
main.c:30:58: error: request for member 'y' in something not a structure or union
   30 |                         scanf("%lf,%lf",&pin[i].x,&pin[i].y);
      |                                                          ^
main.c:51:85: error: request for member 'x' in something not a structure or union
   51 |                                                         len[h] = sqrt(pow(p[(h+1)%3].x-p[h].x,2)+pow(p[(h+1)%3].y-p[h].y,2));
      |                                                                                     ^
main.c:51:92: error: request for member 'x' in something not a structure or union
   51 |                                                         len[h] = sqrt(pow(p[(h+1)%3].x-p[h].x,2)+pow(p[(h+1)%3].y-p[h].y,2));
      |                                                                                            ^
main.c:51:112: error: request for member 'y' in something not a structure or union
   51 |                                                         len[h] = sqrt(pow(p[(h+1)%3].x-p[h].x,2)+pow(p[(h+1)%3].y-p[h].y,2));
      |                                                                                                                ^
main.c:51:119: error: request for member 'y' in something not a structure or union
   51 |                                                         len[h] = sqrt(pow(p[(h+1)%3].x-p[h].x,2)+pow(p[(h+1)%3].y-p[h].y,2));
      |                                                                                                                       ^
main.c:60:81: error: request for member 'x' in something not a structure or union
   60 |                                                         len[3+h] = sqrt(pow(p[3].x-p[h].x,2)+pow(p[3].y-p[h].y,2));
      |                                                                                 ^
main.c:60:88: error: request for member 'x' in something not a structure or union
   60 |                                                         len[3+h] = sqrt(pow(p[3].x-p[h].x,2)+pow(p[3].y-p[h].y,2));
      |                                                                                        ^
main.c:60:102: error: request for member 'y' in something not a structure or union
   60 |                                                         len[3+h] = sqrt(pow(p[3].x-p[h].x,2)+pow(p[3].y-p[h].y,2));
      |                                                                                                      ^
main.c:60:109: error: request for member 'y' in something not a structure or union
   60 |                                                         len[3+h] = sqrt(pow(p[3].x-p[h].x,2)+pow(p[3].y-p[h].y,2));
      |                                                                                                             ^