Language: C

#include<stdio.h>
#include<stdlib.h>
#include<math.h>
#include<string.h>
/*
三辺a,b,c
s=(a+b+c)/2
S={s(s-a)(s-b)(s-c)}^(1/2)

*/

double check(double a,double b,double c){
	double s,S;
	
	s=(a+b+c)/2;
	
	S=sqrt(s*(s-a)*(s-b)*(s-c));
	
	return S;
}
	

int main(void){
	double x[4],y[4],s,t,a,b,c,s1,s2;
	int i,l,f;
	char c1[20],c2[20];
	
	while((scanf("%lf,%lf,%lf,%lf,%lf,%lf,%lf,%lf",&x[0],&y[0],&x[1],&y[1],&x[2],&y[2],&x[3],&y[3]))!=EOF){
		//for(i=0;i<4;i++)scanf("%lf,%lf,",&x[i],&y[i]);
		for(i=0;i<20;i++){
			c1[i]='NULL';
			c2[i]='NULL';
		}
		//scanf("%lf,%lf,%lf,%lf,%lf,%lf,%lf,%lf",&x[0],&y[0],&x[1],&y[1],&x[2],&y[2],&x[3],&y[3]);
		s1=s2=0;
		//点acで区切った三角形
		//+b
		a=sqrt(pow((x[0]-x[2]),2)+pow((y[0]-y[2]),2));
		b=sqrt(pow((x[0]-x[1]),2)+pow((y[0]-y[1]),2));
		c=sqrt(pow((x[1]-x[2]),2)+pow((y[1]-y[2]),2));
		s1+=check(a,b,c);
		
		//+d
		a=sqrt(pow((x[0]-x[2]),2)+pow((y[0]-y[2]),2));
		b=sqrt(pow((x[0]-x[3]),2)+pow((y[0]-y[3]),2));
		c=sqrt(pow((x[3]-x[2]),2)+pow((y[3]-y[2]),2));
		s1+=check(a,b,c);
		
		
		//点bdで区切った三角形
		//+a
		a=sqrt(pow((x[1]-x[3]),2)+pow((y[1]-y[3]),2));
		b=sqrt(pow((x[0]-x[1]),2)+pow((y[0]-y[1]),2));
		c=sqrt(pow((x[0]-x[3]),2)+pow((y[0]-y[3]),2));
		s2+=check(a,b,c);
		
		//+c
		a=sqrt(pow((x[1]-x[3]),2)+pow((y[1]-y[3]),2));
		b=sqrt(pow((x[2]-x[1]),2)+pow((y[2]-y[1]),2));
		c=sqrt(pow((x[2]-x[3]),2)+pow((y[2]-y[3]),2));
		s2+=check(a,b,c);
		
		sprintf(c1,"%lf",s1);
		sprintf(c2,"%lf",s2);
		
		if(strcmp(c1,c2)==0){
			printf("YES\n");
			continue;
		}
		else{
			printf("NO\n");
			break;
		}
		
	}
	
	return 0;
}
	
	
	

Compiler output:
main.c: In function 'main':
main.c:31:31: warning: multi-character character constant [-Wmultichar]
   31 |                         c1[i]='NULL';
      |                               ^~~~~~
main.c:31:31: warning: overflow in conversion from 'int' to 'char' changes value from '1314212940' to '76' [-Woverflow]
main.c:32:31: warning: multi-character character constant [-Wmultichar]
   32 |                         c2[i]='NULL';
      |                               ^~~~~~
main.c:32:31: warning: overflow in conversion from 'int' to 'char' changes value from '1314212940' to '76' [-Woverflow]
/usr/bin/ld: /tmp/ccCvumlz.o: in function `check':
main.c:(.text+0x71): undefined reference to `sqrt'
/usr/bin/ld: /tmp/ccCvumlz.o: in function `main':
main.c:(.text+0xfa): undefined reference to `pow'
/usr/bin/ld: main.c:(.text+0x12f): undefined reference to `pow'
/usr/bin/ld: main.c:(.text+0x14a): undefined reference to `sqrt'
/usr/bin/ld: main.c:(.text+0x17c): undefined reference to `pow'
/usr/bin/ld: main.c:(.text+0x1b1): undefined reference to `pow'
/usr/bin/ld: main.c:(.text+0x1cc): undefined reference to `sqrt'
/usr/bin/ld: main.c:(.text+0x1fe): undefined reference to `pow'
/usr/bin/ld: main.c:(.text+0x233): undefined reference to `pow'
/usr/bin/ld: main.c:(.text+0x24e): undefined reference to `sqrt'
/usr/bin/ld: main.c:(.text+0x2ae): undefined reference to `pow'
/usr/bin/ld: main.c:(.text+0x2e3): undefined reference to `pow'
/usr/bin/ld: main.c:(.text+0x2fe): undefined reference to `sqrt'
/usr/bin/ld: main.c:(.text+0x330): undefined reference to `pow'
/usr/bin/ld: main.c:(.text+0x365): undefined reference to `pow'
/usr/bin/ld: main.c:(.text+0x380): undefined reference to `sqrt'
/usr/bin/ld: main.c:(.text+0x3b2): undefined reference to `pow'
/usr/bin/ld: main.c:(.text+0x3e7): undefined reference to `pow'
/usr/bin/ld: main.c:(.text+0x402): undefined reference to `sqrt'
/usr/bin/ld: main.c:(.text+0x462): undefined reference to `pow'
/usr/bin/ld: main.c:(.text+0x497): undefined reference to `pow'
/usr/bin/ld: main.c:(.text+0x4b2): undefined reference to `sqrt'
/usr/bin/ld: main.c:(.text+0x4e4): undefined reference to `pow'
/usr/bin/ld: main.c:(.text+0x519): undefined reference to `pow'
/usr/bin/ld: main.c:(.text+0x534): undefined reference to `sqrt'
/usr/bin/ld: main.c:(.text+0x566): undefined reference to `pow'
/usr/bin/ld: main.c:(.text+0x59b): undefined reference to `pow'
/usr/bin/ld: main.c:(.text+0x5b6): undefined reference to `sqrt'
/usr/bin/ld: main.c:(.text+0x616): undefined reference to `pow'
/usr/bin/ld: main.c:(.text+0x64b): undefined reference to `pow'
/usr/bin/ld: main.c:(.text+0x666): undefined reference to `sqrt'
/usr/bin/ld: main.c:(.text+0x698): undefined reference to `pow'
/usr/bin/ld: main.c:(.text+0x6cd): undefined reference to `pow'
/usr/bin/ld: main.c:(.text+0x6e8): undefined reference to `sqrt'
/usr/bin/ld: main.c:(.text+0x71a): undefined reference to `pow'
/usr/bin/ld: main.c:(.text+0x74f): undefined reference to `pow'
/usr/bin/ld: main.c:(.text+0x76a): undefined reference to `sqrt'
collect2: error: ld returned 1 exit status