#include <iostream>
#include <cstdlib>
#include <cmath>
using namespace std;
int main()
{
    float a,b,c,x,em;
    cin>>a;
    cin>>b;
    cin>>c;
    x= (a+b+c)/2;
    em= sqrt(x*(x-a)*(x-b)*(x-c));
    cout<<em<<endl;
    
    



    system("PAUSE");
    return 0;
}