import java.util.Scanner;

public class TriangularArea {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        double a = sc.nextDouble();
        double b = sc.nextDouble();
        double c = sc.nextDouble();
        double p = (a + b + c) / 2.0;
        double result = Math.sqrt(p * (p - a) * (p - b) * (p - c));
        if((int)(result * 100) % 10 < 5){
            result = (int)(result * 10) / 10.0;
            System.out.println(result);
        }
        else{
            result = (int)(result *10) / 10.0 + 0.1;
            System.out.println(result);
        }
    }
}