#!/usr/bin/env python3
#fileencoding: utf-8

N = int(input())
A = [int(i) for i in input().strip().split(" ")]

def is_kad(a,b,c):
    l = sorted([a,b,c])
    if (l[1] == a or l[1] == c) and (a != b and b != c and c != a):
        return True
    return False

count = 0
for i in range(N-2):
    if is_kad(A[i],A[i+1],A[i+2]):
        count += 1
print(count)