#include <bits/stdc++.h>
using namespace std;

using ld = long double;

constexpr ld EPS = 1e-8;
constexpr ld PI = acos(-1);

ld sqr(ld a) { return a*a; }

// Inscribe the given quadrilateral into a circle of the given radius.
// Returns {radius too large (true) or small (false), resulting area}
pair<bool, ld> inscribe(const vector<ld> lns, ld rad) {
	const size_t n = lns.size();
	ld ang = 0.0, area = 0.0;

	// Inscribe the first n-1 lengths.
	for (size_t i = 0; i + 1 < n; ++i) {
		ang += 2 * asin(lns[i] / 2 / rad);
		area += (lns[i] / 2) * sqrt(abs(sqr(lns[i] / 2) - sqr(rad)));
	}

	// Inscribe the last length depending on whether we cover the center.
	ld angn = 2 * asin(lns[n-1] / 2 / rad);
	ld arean = (lns[n-1] / 2) * sqrt(abs(sqr(lns[n-1] / 2) - sqr(rad)));

	if (ang > PI) {
		// Cover the center.
		ang += angn;
		area += arean;
		return {ang < 2 * PI, area};
	} else {
		// Do not cover the center.
		ang -= angn;
		area -= arean;
		return {ang > 0, area};
	}
}

int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);

	vector<ld> lns;
	ld ln;
	while (cin >> ln) lns.push_back(ln);
	sort(lns.begin(), lns.end());

	// Binary search the radius of the circle in which we
	// can inscribe the optimal quadrilateral.
	ld l = lns.back() / 2, r = l;
	while (!inscribe(lns, r).first) r *= 2;

	while(r > (1L + 1e-20L)*l){
		ld m = (l + r) / 2;
		if (inscribe(lns, m).first)
			r = m;
		else
			l = m;
	}

	printf("%.20lf\n", double(inscribe(lns, (l+r)/2).second));

	return 0;
}