class Program
    {
        static void Main(string[] args)
        {
            const int rangeStart = 206938;
            const int rangeEnd = 679128;
            var result = Enumerable.Range(rangeStart, rangeEnd - rangeStart).Select(MatchesCriteria);

            Console.WriteLine($"Part 1 Possible passwords: {result.Count(r => r.Item1)}");
            Console.WriteLine($"Part 2 Possible passwords: {result.Count(r => r.Item2)}");
        }

        static (bool, bool) MatchesCriteria(int n)
        {
            var exactPair = false;
            var runLength = 0;
            var maxRunLength = 0;
            var prev = 11;

            while (n >= 10)
            {
                var least = n % 10;

                if (!Check(prev, least, ref runLength, ref maxRunLength, ref exactPair))
                    return (false, false);

                n /= 10;
                prev = least;
            }

            if (!Check(prev, n, ref runLength, ref maxRunLength, ref exactPair))
                return (false, false);

            if (runLength == 2)
                exactPair = true;

            return (maxRunLength >= 2, exactPair);
        }

        static bool Check(int prev, int x, ref int runLength, ref int maxRunLength, ref bool exactPair)
        {
            if (x == prev)
            {
                maxRunLength = Math.Max(++runLength, maxRunLength);
            }
            else
            {
                if (!exactPair)
                {
                    exactPair = runLength == 2;
                }
                runLength = 1;
            }

            return !(prev < x);
        }
    }