#include<iostream>
#include<algorithm>
#include<cstdio>
#include<cstdlib>
#include<vector>
#include<map>
#include<queue>
#include<string>
#include<sstream>
#include<cmath>
#include<numeric>
using namespace std;

int main(){

  int n;
  cin >> n;
  vector<int> a(n);
  for(int i=0; i<n; i++) cin >> a[i];

  int cnt = 0;
  for(int i=1; i<n-1; i++){
    if( a[i-1] > a[i] && a[i] > a[i+1] ) continue;
    if( a[i-1] < a[i] && a[i] < a[i+1] ) continue;
    if( a[i-1] == a[i+1] || a[i] == a[i+1] || a[i] == a[i-1] ) continue;

    cnt++;
  }
  cout << cnt << endl;

  return 0;
}