<?php
$N = trim(fgets(STDIN));
$A = explode(" ", trim(fgets(STDIN)));
$ans = 0;
for($i = 0; $i < $N - 2; $i++) {
  $a = $A[$i];
  $b = $A[$i + 1];
  $c = $A[$i + 2];
  if(($a != $b) && ($a != $c) && ($b != $c)) {
    if((($b < $a) && ($b < $c)) || (($b > $a) && ($b > $c))) {
      $ans++;
    } 
  }
}
print($ans);
?>