#include <algorithm>
#include <cstdio>
#include <iostream>
#include <string>
#include <vector>
#include <map>
#define rep(i, n) for (int i=0; i<n; ++i)
#define FOR(i, a, b) for (int i=a; i<b; ++i)
#define pb(x) push_back(x)
#define ll long long
#define ul unsigned long long
#define debug(x) cout  << #x << " : " << x << endl;
using namespace std;

const int Inf =1500000000;
int main () {
  int n;
  cin >> n;
  int a[n];
  rep(i, n) cin >> a[i];
  int cnt = 0;
  rep(i, n - 2) {
    if (a[i] == a[i+2]);
    else if (a[i+1] > a[i] && a[i + 1] > a[i+2]) {
      cnt++;
    } else if (a[i+1] < a[i] && a[i + 1] < a[i+2]){
      cnt++;
    }
  }
  
   cout << cnt << endl;
   return 0;
}