C :

#include <stdio.h>
#include <math.h>
int main()
{
	int a,b,c;
	double area,p;
	scanf("%d %d %d",&a,&b,&c);
    p=(a+b+c)/2.0;
	if((a+b>c)&&(a+c>b)&&(b+c>a))
	{area=sqrt(p*(p-a)*(p-b)*(p-c));
	printf("%.3f",area);}
	printf("\n");
	return 0;
}

C++ :

#include <stdio.h>
#include <math.h>
#define calcS(a, b, c) (0.5 * ((a) + (b) + (c)))
#define calcArea(a, b, c) \
	(sqrt(calcS(a, b, c) * \
	(calcS(a, b, c) - (a)) * \
	(calcS(a, b, c) - (b)) * \
	(calcS(a, b, c) - (c))))
int main() {
	int a, b, c;
	scanf("%d%d%d", &a, &b, &c);
	printf("%.3f\n", calcArea(a, b, c));
	return 0;
}

Pascal :

var
  a,b,c:longint;
  s,d:double;
begin
  readln(a,b,c);
  d:=(a+b+c)/2;
  s:=sqrt(d*(d-a)*(d-b)*(d-c));
  writeln(s:0:3);
end.

Java :

import java.text.DecimalFormat;
import java.util.Scanner;

public class Main {
  static DecimalFormat df = new DecimalFormat("0.000") ; 
  public static void main(String[] args) {
	  Scanner s = new Scanner(System.in) ; 
	  int a = s.nextInt() ;
	  int b = s.nextInt() ;
	  int c = s.nextInt() ;
	  
	  if(a+b>c&&a+c>b&b+c>a){
		  System.out.println(f(a, b, c)) ;
	  }
   }
   
   static String f(int a , int b , int c){
	   double s = ((double)a+(double)b+(double)c)/2 ;
	   
	   double area = Math.sqrt(s*(s-a)*(s-b)*(s-c)) ;
	   
	   return df.format(area) ;
   }
}