12.AssociativeArrays-Student Academy

12.Student Academy
Write a program that keeps information about students and their grades.
You will receive n pair of rows. First you will receive the student's name, after that you will receive his grade. Check if the student already exists and if not, add him. Keep track of all grades for each student.
When you finish reading the data, keep the students with average grade higher than or equal to 4.50. Order the filtered students by average grade in descending order.
Print the students and their average grade in the following format:
{name} –> {averageGrade}
Format the average grade to the 2nd decimal place.

Examples
Input                   Output                    Input                   Output
5                       John -> 5.00              5                       Robert -> 6.00
John                    George -> 5.00            Amanda                  Rob -> 5.50
5.5                     Alice -> 4.50             3.5                     Christian -> 5.00
John                                              Amanda
4.5                                               4
Alice                                             Rob
6                                                 5.5
Alice                                             Christian
3                                                 5
George                                            Robert
5                                                 6

using System;
using System.Collections.Generic;
using System.Linq;

namespace 12StudentAcademy
{
    class Program
    {
        static void Main(string[] args)
        {
            //int line = int.Parse(Console.ReadLine());
            //var students = new Dictionary<string, List<double>>();

            //for (int i = 0; i < line; i++)
            //{
                //string student = Console.ReadLine();
                //double grade = double.Parse(Console.ReadLine());                           //Друго решение.

                //if (!students.ContainsKey(student))
                    //students[student] = new List<double>();

                //students[student].Add(grade);
            //}

            //Console.WriteLine(string.Join($"{Environment.NewLine}",students
                //.Where(x => (x.Value.Sum() / x.Value.Count) >= 4.50)
                //.OrderByDescending(x => x.Value.Sum() / x.Value.Count)
                //.Select(x => $"{x.Key} -> {x.Value.Sum() / x.Value.Count():f2}")));

            var studentAndGrades = new Dictionary<string, List<decimal>>();
            var averageGrades = new Dictionary<string, decimal>();
            var numPair = int.Parse(Console.ReadLine());
            for (int i = 1; i <= numPair; i++)
            {
                var name = Console.ReadLine();
                var grade = decimal.Parse(Console.ReadLine());
                if (!studentAndGrades.ContainsKey(name))
                {
                    studentAndGrades.Add(name, new List<decimal> { grade });                     //Мое решение.
                }
                else
                {
                    studentAndGrades[name].Add(grade);
                }
            }
            foreach (var item in studentAndGrades)
            {
                var averageGrade = studentAndGrades[item.Key].Average();
                averageGrades.Add(item.Key, averageGrade);
            }
            foreach (var item in averageGrades.OrderByDescending(x => x.Value))
            {
                if (item.Value >= 4.50m)
                {
                    Console.WriteLine($"{item.Key} -> {item.Value:F2}");
                }
            }
        }
    }
}