using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var score = 73;

​

        // Compare for low to high

        if (score < 50) {

            Console.WriteLine("F");

        } else if (score < 60) {

            Console.WriteLine("D");

        } else if (score < 70) {

            Console.WriteLine("C");

        } else if (score < 80) {

            Console.WriteLine("B");

        } else {

            Console.WriteLine("A");

        }

        // Compare for high to low

        if (score >= 80) {

            Console.WriteLine("A");

        } else if (score >= 70) {

            Console.WriteLine("B");

        } else if (score >= 60) {

            Console.WriteLine("C");

        } else if (score >= 50) {

            Console.WriteLine("D");

        } else {

            Console.WriteLine("F");

        }

        // Compare for high to low with stepping dictionary

        var scoreSteps = new Dictionary<int, string>

        {

            { 80, "A" },

            { 70, "B" },

            { 60, "C" },

            { 50, "D" },

        };

        var isF = true;

        foreach (var kv in scoreSteps) {

            var baseScore = kv.Key;

            var grade = kv.Value;

            if (score >= baseScore) {

                isF = false;

                Console.WriteLine(grade);

                break;

            }

        }

        if (isF) {

            Console.WriteLine("F");

        }

    }

}