class Program
{
    static void Main(string[] args)
    {
        string inputCount = Console.ReadLine()!;
        double count = double.Parse(inputCount);
        double expectedValue = (1.0 + 2.0 + 3.0 + 4.0 + 5.0 + 6.0) / 6.0;
        
        Console.WriteLine(expectedValue * count);
    }
}