Excessive number is a number for which the sum of its proper divisors is greater than the number itself.

The integer 12 is the first abundant number. Its proper divisors are 1, 2, 3, 4 and 6 for a total of 16.

Solution:

using System;

namespace ExcessiveNumber
{
    class Program
    {
        static void Main()
        {
            Console.WriteLine("Enter the number:");
            var n = Convert.ToInt32(Console.ReadLine());
            var result = ExcessiveSum(n) > n;
            Console.WriteLine(result ? "Excessive  number." : "Not Excessive  number.");
        }

        static int ExcessiveSum(int n)
        {
            var sum = 0;
            for (var i = 1; i <= Math.Sqrt(n); i++)
            {
                if (n % i != 0) continue;

                if (n / i == i)
                {
                    sum += i;
                }
                else
                {
                    sum += i;
                    sum += n / i;
                }
            }

            sum -= n;
            return sum;
        }
    }
}