#include <iostream>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <vector>

using namespace std;
using ldouble = long double;

void solve() {
    vector<ldouble> xs(3);
    for (auto& x : xs) cin >> x;

    auto theta = [&]() {
        return acos((xs[2] * xs[2] - xs[0] * xs[0] - xs[1] * xs[1]) / (xs[0] * xs[1] * 2));
    };

    auto s = xs[0] * xs[1] * sin(theta()) / 2;

    for (int i = 0; i < 3; ++i) {
        s -= xs[0] * xs[1] * sin(theta()) / 8;
        rotate(xs.begin(), xs.begin() + 1, xs.end());
    }

    cout << s << "\n";
}

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);

    solve();

    return 0;
}