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