#include<iostream>
#include<cstdio>
#include<string>
#include<bits/stdc++.h>
#define LL long long
using namespace std;
const int INF=0x3f3f3f3f;
const int N=2e5+10;
double a,b,c;
int main(){
	cin>>a>>b>>c;
	double p=(a+b+c)/2;
	printf("%.2f",sqrt(p*(p-a)*(p-b)*(p-c)));
}