<?php

$ids = explode("\n", file_get_contents('input.txt'));
//$ids = ['FBFBBFFRLR'];

$seat_ids = [];

function decodeRow($row)
{
    $min = 1;
    $max = 128;

    foreach (str_split($row) as $cell) {
        if ($cell === 'F') {
            $max = floor($min + ($max - $min) / 2);
        } else {
            $min = ceil($max - ($max - $min) / 2);
        }
    }

    return $min - 1;
}

function decodeColumn($column)
{
    $min = 1;
    $max = 8;

    foreach (str_split($column) as $cell) {
        if ($cell === 'L') {
            $max = floor($min + ($max - $min) / 2);
        } else {
            $min = ceil($max - ($max - $min) / 2);
        }
    }

    return $min - 1;
}

foreach ($ids as $id) {
    $encoded_row = substr($id, 0, 7);
    $encoded_column = substr($id, 7);
    $row = decodeRow($encoded_row);
    $column = decodeColumn($encoded_column);
    $seat_ids[] = $row * 8 + $column;
}

sort($seat_ids);
for ($i = current($seat_ids), $l = end($seat_ids), reset($seat_ids); $i <= $l; $i++) {
    if (!in_array($i, $seat_ids, true)) {
        echo $i;
    }
}