#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
constexpr int Inf = 2000000030;
constexpr ll INF= 2000000000000000001;
constexpr ll MOD = 1000000007;
const double PI = 3.14159265358979323846;
typedef pair<ll,ll> P;
typedef pair<ll,P> PP;

template<typename T> 
vector<T> make_vector(size_t s) {
    return vector<T>(s);
}

template<typename T,typename... Args>
auto make_vector(size_t s,Args... args) {
    return vector(s,make_vector<T>(args...));
}

template<typename T> inline bool chmax(T &a, T b) {
    if (a < b) {
        a = b;
        return 1;
    }
    return 0;
}

template<typename T> inline bool chmin(T &a, T b) {
    if (a > b) {
        a = b;
        return 1;
    }
    return 0;
}

ll mod(ll val, ll M) {
    val = val % M;
    if(val < 0) {
        val += M;
    }
    return val;
}

template<typename T>
T modpow(T N, T P, T M){
    if(P == 0) return 1;
    if(P < 0) return 0;
    if(P % 2 == 0){
        ll t = modpow(N, P/2, M);
        if(M == -1) return t * t;
        return t * t % M;
    }
    if(M == -1) return N * modpow(N,P - 1,M);
    return N * modpow(N, P-1, M) % M;
}

int main() {
    double A,B,C;
    cin >> A >> B >> C;
    double S = (A + B + C) / 2;
    cout << fixed << setprecision(10) << sqrt(S * (S - A) * (S - B) * (S - C)) / 4 << endl;
}