#include<bits/stdc++.h>
using namespace std;

double f(double a,double b,double c){
    double s=(a+b+c)/2;
    return sqrt(s*(s-a)*(s-b)*(s-c));
}
int main(){
   int a,b,c;
   cin>>a>>b>>c;
   cout<<fixed<<setprecision(1)<<f(a,b,c)<<endl;
   return 0;
}

是的你没看错