Problem: Retrieve the updated value.

I am having trouble retrieving the latest updated value (NetProfit of position) for my bot. I have the following code:

using System;
using cAlgo.API;
using cAlgo.API.Collections;
using cAlgo.API.Indicators;
using cAlgo.API.Internals;
using System.Collections.Generic;
using System.Threading;

namespace cAlgo.Robots
{
    [Robot(AccessRights = AccessRights.None, AddIndicators = true)]
    public class Check : Robot
    {
        [Parameter(DefaultValue = "Hello world!")]
        public string Message { get; set; }

        List<Position> ps;
        protected override void OnStart()
        {
            ps = new List<Position>();
            foreach(var p in Positions)
            {
                ps.Add(p);
            }
        }

        
        protected override void OnTick()
        {
             string log = "";
            foreach (var p in ps)
            {
                log+= $"{p.SymbolName}:{p.NetProfit} ";
            }
            Print(log);
            Thread.Sleep(2000); // As a replacement for algorithmic complexity, the runtime complexity can take up to 2 seconds for analysis.
        }

        protected override void OnStop()
        {
            // Handle cBot stop here
        }
    }
}

The problem is that the longer it runs, the more the position value I receive deviates over time!
 

How can I fix this error?