using System;

class TriangleArea
{
    static void Main()
    {
        Console.WriteLine("How you want to calculate the area of triangle?\n");
        Console.WriteLine("1. By side and altitude.\n2. By three sides\n3. Two sides and an angle between them.");
        int choice = int.Parse(Console.ReadLine());

        switch (choice)
        {
            case 1: SideAndAltitude();
                break;
            case 2: ThreeSides();
                break;
            case 3: TwoSidesAndAngle();
                break;
            default: Console.WriteLine("Enter correct number!");
                break;
        }
    }

    private static void SideAndAltitude()
    {
        Console.WriteLine("Enter a side:");
        double side = double.Parse(Console.ReadLine());
        Console.WriteLine("Enter a altitude:");
        double altitude = double.Parse(Console.ReadLine());
        Console.WriteLine("S = {0}", side * altitude / 2);
    }

    private static void ThreeSides()
    {
        Console.WriteLine("Enter a first side:");
        double sideOne = double.Parse(Console.ReadLine());
        Console.WriteLine("Enter a second side:");
        double sideTwo = double.Parse(Console.ReadLine());
        Console.WriteLine("Enter a third side:");
        double sideThree = double.Parse(Console.ReadLine());
        double s = (sideOne + sideTwo + sideThree) / 2;
        Console.WriteLine("S = {0}", Math.Sqrt(s*(s-sideOne)*(s-sideTwo)*(s-sideThree)));
    }

    private static void TwoSidesAndAngle()
    {
        Console.WriteLine("Enter a first side:");
        double sideOne = double.Parse(Console.ReadLine());
        Console.WriteLine("Enter a second side:");
        double sideTwo = double.Parse(Console.ReadLine());
        Console.WriteLine("Enter an angle:");
        double angle = double.Parse(Console.ReadLine());
        Console.WriteLine("S = {0}", sideOne * sideTwo*Math.Sin(Math.PI*angle/180) / 2);
    }

}