#include<bits/stdc++.h>
using namespace std;
int main(){
    int a,b,c;
    int sum,p;
    cin>>a>>b>>c;
    p=(a+b+c)*0.5;
    sum=sqrt(p*(p-a)*(p-b)*(p-c));
    cout<<setprecision(1)<<sum<<endl;
}