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

using ld = long double;

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

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

	vector<ld> lns;
	ld ln;
	while (cin >> ln) lns.push_back(ln);

	ld l = (*std::max_element(lns.begin(), lns.end())) / 2.0;
	ld r = 1e100;
	while (r - l > EPS) {
		ld m = (l + r) / 2.0, s = 0.0;
		for (ld ln : lns)
			s += 2 * asin(ln / 2 / m);
		cerr << ' ' << l << ' '<< r << " -> " << m << ' ' << s << endl;
		if (s >= 2 * PI)
			l = m;
		else
			r = m;
	}
	cerr << "radius: " << l << endl;

	ld ans = 0.0;
	for (ld ln : lns)
		ans += (ln / 2) * sqrt(abs(pow(ln / 2, 2) - pow(l, 2)));
	printf("%.20lf\n", double(ans));

	return 0;
}