private static string SolveSilver(string input)
        {
            var walker = new CityWalker(input);

            var location = walker.GetLocations().Last();
            var distance = Math.Abs(location.X) + Math.Abs(location.Y);

            return($"We think Easter Bunny HQ is at {location.X},{location.Y} for a total distance of {distance}");
        }
        private static string SolveGold(string input)
        {
            var walker = new CityWalker(input);

            var history = new HashSet <Coordinate>();

            Coordinate found = null;

            foreach (var location in walker.GetLocations())
            {
                if (history.Contains(location))
                {
                    found = location;
                    break;
                }

                history.Add(location);
            }

            var distance = found != null?Math.Abs(found.X) + Math.Abs(found.Y) : 0;

            return($"Easter Bunny HQ is actually at {found?.X},{found?.Y} for a total distance of {distance}");
        }