C++ :

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

Python :

# coding=utf-8
x=float(input())
y=float(input())
z=float(input())
p=(x+y+z)/2
s=(p*(p-x)*(p-y)*(p-z))**0.5
print(format(s,'.2f'))