#include <bits/stdc++.h>
using namespace std;
int a,b,c,p;
double s;
int main(){
	scanf("%d %d %d",&a,&b,&c);
	p=(a+b+c)/2.0;
	s=sqrt(p*(p-a)*(p-b)*(p-c));
	printf("%.1lf",s);
	return 0;
}