/* -*- coding: utf-8 -*-
 *
 * 116.cc: No.116 門松列(1) - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int MAX_N = 100;

/* typedef */

/* global variables */

int as[MAX_N];

/* subroutines */

/* main */

int main() {
  int n;
  cin >> n;
  for (int i = 0; i < n; i++) cin >> as[i];

  int cnt = 0;
  for (int i = 1; i < n - 1; i++) {
    int &al = as[i - 1], &ac = as[i], &ar = as[i + 1];
    if ((ac < al && al < ar) || (ar < al && al < ac) ||
	(ac < ar && ar < al) || (al < ar && ar < ac)) cnt++;
  }

  printf("%d\n", cnt);
  return 0;
}