C :

#include<stdio.h>
#include<math.h>
#define S(x,y,z) (x+y+z)/2;
#define area(x,y,z,S) sqrt(S*(S-x)*(S-y)*(S-z))
main()
{
	double a,b,c,S;
	scanf("%lf%lf%lf",&a,&b,&c);
	S=S(a,b,c);
	printf("%.3f\n",area(a,b,c,S));
}

C++ :

#include<bits/stdc++.h>
using namespace std;
double a,b,c,x;
int main()
{
    cin>>a>>b>>c;
    x=(a+b+c)/2;
    printf("%.3f",sqrt(x*(x-a)*(x-b)*(x-c)));
    return 0;
}