How to make sum of last 30 values on indicator

Hi,

I have simple indicator which calculates some values and prints them on price chart on each bar...some bars don't have values

how to make sum of those values (in last 30 bars), and how to count them as they are not on each bar

my indicator is something like High - Low...so you can use that example to help me ..thanks

using System;
using cAlgo.API;
 
namespace cAlgo.Indicators
{
    [Indicator(AccessRights = AccessRights.None)]
    class HighMinusLow : Indicator
    {
        [Output("Main")]
        public IndicatorDataSeries Result { get; set; }
 
        public override void Calculate(int index)
        {
            Result[index] = MarketSeries.High[index] - MarketSeries.Low[index];
        }
    }
}