Common - Rare

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Practice
{
    class Program
    {
        static void Main(string[] args)
        {
            Random rnd = new Random();
            int common = 0, rare = 0, random, smallest = 0, biggest = 0;
            Console.WriteLine("Specify array length:");
            int length = int.Parse(Console.ReadLine());
            int[] arr = new int[length];
            int[] frequency = new int[length];
            for (int i = 0; i < arr.Length; i++)
            {
                random = rnd.Next(0, 10);
                arr[i] = random;
            }
            for (int i = 0; i < frequency.Length; i++)
            {
                frequency[i] = 1;
                for (int a = 0; a < arr.Length; a++)
                {
                    if (arr[i] == arr[a])
                        frequency[i]++;
                }
            }
            for (int i = 0; i < frequency.Length; i++)
            {
                if (i == 0)
                {
                    smallest = frequency[i];
                    biggest = frequency[i];
                }
                else
                {
                    if (frequency[i] > biggest)
                        biggest = frequency[i];
                    if (frequency[i] < smallest)
                        smallest = frequency[i];
                }
            }
            common = Array.IndexOf(frequency, biggest);
            rare = Array.IndexOf(frequency, smallest);
            Console.WriteLine("Common: {0}\nRare: {1}", arr[common], arr[rare]);
        }
    }
}