<?php

$front_row = 0;
$rear_row = 127;
$cols_on_plane = 8;

$all_seat_ids = [];

$lower_half_rows = 'F';
$upper_half_rows = 'B';

$lower_half_cols = 'L';
$upper_half_cols = 'R';

$highest_seat_id = 0;

$lowest_row = $rear_row;
$highest_row = $front_row;

$fp = fopen("input.txt", "r");

while ( $line = trim(fgets($fp)) )
{	
	$low = 0;
	$high = $rear_row;
			
	for ( $i = 0; $i < 7; ++$i )
	{
		if ( $line{$i} == $lower_half_rows )
		{
			$high = floor($low + (($high - $low) / 2));			
		}
		else
		{
			$low = ceil($high - (($high - $low) / 2));
		}
	}		
	
	assert($low == $high);
	$row = $low;

	if ( $row > $highest_row )
	{
		$highest_row = $row;
	}
	else if ( $row < $lowest_row )
	{
		$lowest_row = $row;
	}
	
	//
	
	$low = 0;
	$high = $cols_on_plane - 1;
	
	for ( $i = 7; $i < 10; ++$i )
	{
		if ( $line{$i} == $lower_half_cols )
		{
			$high = floor($low + (($high - $low) / 2));			
		}
		else
		{
			$low = ceil($high - (($high - $low) / 2));
		}
	}
	
	assert($low == $high);
	$col = $low;
		
	//
	
	$seat_id = ($row * 8) + $col;
	$all_seat_ids[] = $seat_id;
	
	if ( $seat_id > $highest_seat_id )
	{
		$highest_seat_id = $seat_id;
	}
}
	
fclose($fp);

sort($all_seat_ids);

for ( $i = 0; $i < count($all_seat_ids); ++$i )
{
	$curr = $all_seat_ids[$i];
	$next = $all_seat_ids[$i + 1];
	
	if ( $curr + 1 != $next )
	{
		print ($curr + 1) . "\n";
		break;
	}
}