C :

#include<stdio.h>
#include<math.h>
#define S(a,b,c) (a+b+c)/2
#define AREA(s,a,b,c) sqrt(s*(s-a)*(s-b)*(s-c))
int main()
{
    float a,b,c,s,area;
    scanf("%f%f%f",&a,&b,&c);
    s=S(a,b,c);
    area=AREA(s,a,b,c);
    printf("%.3f\n",area);
    return 0;
}

C++ :

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