<?php
$str = trim(fgets(STDIN));
$act = 0;
$lastA = strrpos($str, 'A');
$firstB = strpos($str, 'B');
if($lastA === FALSE || $firstB === FALSE) { echo $act.PHP_EOL; }
while($lastA > $firstB) {
    $len = strlen($str);
    $pos = $firstB;
    while($pos < $lastA) {
        if($str[$pos] == $str[$pos+1]) { $pos++; continue; }
        $temp = $str[$pos];
        $str[$pos] = $str[$pos+1];
        $str[$pos+1] = $temp;
        $act++;
        $pos++;
    }
    $lastA = strrpos($str, 'A');
    $firstB = strpos($str, 'B');
}
echo $act.PHP_EOL;