using System;

using System.Linq;

public class Program

{

    public class MonthlyStats

    {

        public DateTime Date { get; set; }

        public ulong FriendCount { get; set; }

        public ulong ScoreCount { get; set; }

    }

​

    public sealed class Person

    {

        public Person(int id, string name, DateTime registrationDate, ulong friendCount, ulong scoreCount)

        {

            Id = id;

            Name = name;

            RegistrationDate = registrationDate;

            FriendCount = friendCount;

            ScoreCount = scoreCount;

        }

​

        public int Id { get; init; }

​

        public string Name { get; set; }

​

        public DateTime RegistrationDate { get; set; }

​

        public ulong FriendCount { get; set; }

​

        public ulong ScoreCount { get; set; }

    }

    public static void Main()

    {

        Person[] persons =

        {

            new(1, "A", new DateTime(2022, 1, 4), 9, 23),

            new(2, "B", new DateTime(2021, 4, 8), 7, 29),

            new(3, "C", new DateTime(2021, 9, 11), 4, 13),

            new(4, "D", new DateTime(2021, 11, 1), 10, 14),

            new(5, "E", new DateTime(2021, 4, 17), 12, 17),

            new(6, "F", new DateTime(2022, 1, 27), 5, 11),

            new(7, "G", new DateTime(2021, 9, 2), 13, 31),

        };

​

        MonthlyStats[] monthlyStats = persons

            .GroupBy(p => new DateTime(p.RegistrationDate.Year, p.RegistrationDate.Month, 1),

                ( firstDayOfMonth, personsInMonth ) => new MonthlyStats

                    {

                        Date = firstDayOfMonth,

                        FriendCount = (ulong)personsInMonth.Sum(p => (decimal)p.FriendCount),

                        ScoreCount = (ulong)personsInMonth.Sum(p => (decimal)p.ScoreCount)

                    })

            .OrderBy(stat => stat.Date)

            .ToArray();

​

        foreach (var stat in monthlyStats)

        {

            Console.WriteLine($"{stat.Date:dd/MM/yyyy}  |  {stat.FriendCount}  |  {stat.ScoreCount}");

        }

    }

}