import java.util.Scanner;

public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
		int b = sc.nextInt();
		int c = sc.nextInt();
		sc.close();

		double s = (a + b + c) / 2.0;
		double x = Math.sqrt(s * (s - a) * (s - b) * (s - c));
		System.out.println(x / 4);
	}
}