Digits sum

Given a non-negative number, write a method that returns sum of its digits.
Expected input and output
DigitsSum(5434) → 16 DigitsSum(904861) → 28
using System;

namespace CSharpExercises
{
    class DigitsSumTask
    {        
        public static int DigitsSum(uint number)
        {
            int sum = 0;
            int i = 10;
            int j = 1;

            while (number / j >= 1)
            {
                sum += (int)(number % i / j);

                i *= 10;
                j *= 10;
            }

            return sum;
        }

        static void Main(string[] args)
        {
            Console.WriteLine(DigitsSum(5));                   // 5
            Console.WriteLine(DigitsSum(1029584739)); // 48
            Console.WriteLine(DigitsSum(99999999));    // 72
        }
    }
}