#include <algorithm>
#include <cstdio>
#include <iostream>
#include <iomanip>
#include <map>
#include <cmath>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <vector>
#include <stdlib.h>
#include <stdio.h>
#include <bitset>
using namespace std;
#define FOR(I,A,B) for(int I = (A); I < (B); ++I)

int main(){
  int n;
  cin >> n;
  int a[n];
  FOR(i,0,n){
    cin >> a[i];
  }
  int ans = 0;
  FOR(i,0,n-2){
    if(a[i] == a[i+2]) continue;
    if(a[i] > a[i+1] && a[i+1] < a[i+2]) ans++;
    if(a[i] < a[i+1] && a[i+1] > a[i+2]) ans++;
  }
  cout << ans << endl;
  return 0;
}