//GIVE ME AC!!!!!!!!!!!!!!!!!
#pragma GCC target("avx")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include<bits/stdc++.h>
#define ll long long
#define floatset() fixed<<setprecision(15)
#define all(n) n.begin(),n.end()
#define rall(n) n.rbegin(),n.rend()
#define rep(i, s, n) for (ll i = s; i < (ll)(n); i++)
using namespace std;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};
const ll inf =1e18;
const ll MOD=1000000007;
const ll mod=998244353;
const double pi=acos(-1);
using P=pair<ll,ll>;
int main(){
    double a,b,c;
    cin>>a>>b>>c;
    double S;
    double s=(a+b+c)/2;
    S=sqrt(s*(s-a)*(s-b)*(s-c));
    cout<<floatset()<<S/4<<endl;
}