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