AdventOfCodeDay20.cs

using System;
using System.Collections.Generic;

namespace AdventOfCodeDay20
{
	class MainClass
	{
		public static void Main (string[] args)
		{
			var input = 36000000;
			var houseNumber = 830000;
			var presents = 0;
			var max = 0;
			do {
				presents = PresentCountForHousePart2 (houseNumber);
				max = Math.Max(presents, max);
				//Console.WriteLine ($"House #{houseNumber}: {presents}");
				if(houseNumber%1000 == 0)
					Console.WriteLine($"house #{houseNumber} presents: {presents} max: {max}");
				houseNumber++;
				if(houseNumber == 20) break;
			} while(presents < input);

			Console.WriteLine ($"House #{--houseNumber} Presents: {presents}");
		}

		public static int PresentCountForHouse(int houseNumber)
		{
			var total = houseNumber * 10;;
			var max = houseNumber/2;
			for (var i = 1; i <= max; i++) {
				if (houseNumber % i == 0) {
					total += (i * 10);
				}
			}
			return total;
		}

		public static int PresentCountForHousePart2(int houseNumber)
		{
			var total = houseNumber * 11;
			var max = houseNumber/2;
			var start = (int)(houseNumber / 50) + 1;
			for (var i = start; i <= max; i++) {
				if (houseNumber % i == 0) {
					total += (i * 11);
				}
			}
			return total;
		}
	}
}