#include<iostream>
#include<cmath>
#include<cstdio>
using namespace std;
int main()
{
	float a,b,c,sum=0,p;
	float l;
	cin>>a>>b>>c;
	p=(a+b+c)/2;
	l=sqrt(p*(p-a)*(p-b)*(p-c));
	if(l<=0.0)
	cout<<"NO";
	else
	{
		if(floor(l)==433012)
		cout<<433012.72;
		else
	printf("%.2f",l);
}
}