#!/usr/bin/env perl

use strict;
use warnings;

my ($n, $k) = split / /, <>; chomp $k;
if ($k <= 0 || $k > $n) {
  print 0;
} else {
  if ($k * 2 == $n + 1) {
    print $n - 1;
  } else {
    print $n - 2;
  }
}