#include<bits/stdc++.h>
using namespace std;
int main()
{	
	double a,b,c;
	cin>>a>>b>>c;
	double p=(a+b+c)/2;
	double d=p-a;
	double e=p-b;
	double f=p-c;
	printf("%.1f",sqrt(p*d*e*f));
	return 0;
}