#include<bits/stdc++.h>
using namespace std;
int main()
{
	while (1) 
	{
		double A, B, C;
		cin >> A >> B >> C;
		double dl = 2 * B * B + (2 * C * C) - (A * A);
		double dq = 2 * A * A + (2 * C * C) - (B * B);
		double dr = 2 * A * A + (2 * B * B) - (C * C);
		if (dl <= 0 || dr <= 0 || dq <= 0)
		{
			cout << "-1.000";
			break;
		}
		double a = 2 * sqrt(dl) / 3;
		double b = 2 * sqrt(dq) / 3;
		double c = 2 * sqrt(dr) / 3;
		double p = (a + b + c) / 2;
		double ans = sqrt(p * (p - a) * (p - b) * (p - c));
		cout << std::fixed << std::setprecision(3) << ans << endl;
	}
	cout << endl ;
	return 0;
}