#include <stdio.h>
#include <atcoder/all>
#include <bits/stdc++.h>
using namespace std;
using namespace atcoder;
using mint = modint998244353;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 1000000000

int main(){
	
	double a,b,c;
	cin>>a>>b>>c;
	double s = (a+b+c)/2.0;
	
	double ans = s;
	ans *= s-a;
	ans *= s-b;
	ans *= s-c;
	
	ans = sqrt(ans);
	ans /= 4.0;
	
	cout<<fixed<<setprecision(10)<<ans<<endl;
	
    return 0;
}