#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> iint;
typedef pair<ll,ll> llll;
#define ALL(x) (x).begin(),(x).end()
const ll zero = 0;
const ll one = 1;
const ll INF = 9223372036854775807; //10^18
const int inINF = 2147483647; //10^9
const ll MOD = 1000000007; //10^9+7
const ll MOD2 = 998244353;
void Yes() {printf("Yes\n");}
void No() {printf("No\n");}
void YES() {printf("YES\n");}
void NO() {printf("NO\n");}

int main(){
    double a, b, c;
    cin >> a >> b >> c;
    double s = (a+b+c) / 2.0;
    double S = sqrt(s*(s-a)*(s-b)*(s-c));
    printf("%32.18f\n", S/4);

}