Letterbox count

Check out how many letterboxes can be reached

EC1EC2EC3EC4E2E3E1E4E5E6E7E8E9E10E11E12E13E14E15E16E17E18N1N2N3N4N5N6N7N8N9N10N11N12N13N14N15N16N17N18N19N20N21N22NW1NW2NW3NW4NW5NW6NW7NW8NW9NW10NW11W1W2W3W4W5W6W7W8W9W10W11W12W13W14SW1SW2SW3SW4SW5SW6SW7SW8SW9SW10SW11SW12SW13SW14SW15SW16SW17SW18SW19SW20SE1SE2SE3SE4SE5SE6SE7SE8SE9SE10SE11SE12SE13SE14SE15SE16SE17SE18SE19SE20SE21SE22SE23SE24SE25SE26SE27SE28WC1WC2