#include<iostream>
#include<iomanip>
using namespace std;
int main()
{
    int a,b,c;   
    cin>>a>>b>>c;
    double p = (a+b+c)/2;
    if(a+b > c)
    {
    double S = sqrt(p*(p-a)*(p-b)*(p-c));
    cout<< fixed <<setprecision(1) << S <<endl;
    }
}