C++ :

#include<iostream>
#include<cmath>
#include<cstdio>
#define S(a,b,c)	(a+b+c)/2
#define AREA(a,b,c)	sqrt(S(a,b,c)*(S(a,b,c)-a)*(S(a,b,c)-b)*(S(a,b,c)-c))
using namespace std;
int main()
{
	double a,b,c;
	cin>>a>>b>>c;
	printf("%.3lf\n",AREA(a,b,c));
	return 0;
}

Pascal :

var
    a,b,c,p,s:real;
begin
    read(a,b,c);
    if ((a+b)>c) and ((b+c)>a) and ((a+c)>b) then begin
        p:=(a+b+c)/2;
        s:=sqrt(p*(p-a)*(p-b)*(p-c));
        write(s:0:3);
    end;
end.