#include<iostream>
#include<cstdio>
#include<cmath>
using namespace std;
float fac(float a,float b,float c)
{
    float p;
    p=(a+b+c)/2.0;
	p=sqrt(p*(p-a)*(p-b)*(p-c));
	return p;
} 
int main()
{
	float m,n,g,x;
	cin>>m>>n>>g;
	cout<<fac(m,n,g)*1.0;
}