;-), Demo:

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

namespace cardsGame
{
    class Program
    {
        static void Main(string[] args)
        {
            List<int> player1 = Console.ReadLine().Split().Select(int.Parse).ToList();
            List<int> player2 = Console.ReadLine().Split().Select(int.Parse).ToList();

            // The game is over, when one of the decks is left without any cards.
            while (player1.Count > 0 && player2.Count > 0)
            {
                //The player, who has the bigger card, takes both cards and puts 
                //them at the back of his hand - the second player's card is last, 
                //and the first person's card (the winning one) is before it 
                //(second to last) and the player with the smaller card must remove 
                //the card from his deck.
                if (player1[0] > player2[0])
                {
                    player1.Add(player1[0]);
                    player1.Add(player2[0]);
                    player1.RemoveAt(0);
                    player2.RemoveAt(0);
                }
                else if (player1[0] < player2[0])
                {
                    player2.Add(player2[0]);
                    player2.Add(player1[0]);
                    player2.RemoveAt(0);
                    player1.RemoveAt(0);
                }
                else if (player1[0] == player2[0])
                {
                    // If both players’ cards have the same values - no one wins, 
                    //and the two cards must be removed from the decks.
                    player1.RemoveAt(0);
                    player2.RemoveAt(0);
                }
            }

            //You have to print the winner on the console and the sum of the left cards:
            int totalSum = 0;
            if (player1.Count == 0)
            {
                foreach (var item in player2)
                {
                    totalSum += item;
                }
                Console.WriteLine($"Second player wins! Sum: {totalSum}");
            }
            else if (player2.Count == 0)
            {
                foreach (var item in player1)
                {
                    totalSum += item;
                }
                Console.WriteLine($"First player wins! Sum: {totalSum}");
            }
        }
    }
}