<?php
    list($n, $k) = explode(" ", trim(fgets(STDIN)));
    if($k == 0 || $k >$n){
        echo "0\n";
    }elseif($n % 2 == 1 && ceil($n / 2) == $k){
        echo ($n -1)."\n";
    }else{
        echo ($n -2)."\n";
    }
    

?>