#include <algorithm>
#include <cstdio>
#include <cstdlib>
#include <cctype>
#include <cmath>
#include <iostream>
#include <queue>
#include <list>
#include <map>
#include <numeric>
#include <set>
#include <sstream>
#include <string>
#include <vector>
using namespace std;
#define REP(i,a,n) for(int i=(a); i<(int)(n); i++)
#define rep(i,n) REP(i,0,n)
#define FOR(it,c) for(__typeof((c).begin()) it=(c).begin(); it!=(c).end(); ++it)
#define ALLOF(c) (c).begin(), (c).end()
typedef long long ll;


int main(){
  int N, a;
  vector<int> v;
  cin >> N;
  rep(i,N){
    cin >> a;
    v.push_back(a);
  }

  int ret = 0;
  rep(i,N-2){
    if(v[i]==v[i+1] || v[i]==v[i+2] || v[i+1]==v[i+2]) continue;
    vector<int> w;
    w.push_back(v[i]);
    w.push_back(v[i+1]);
    w.push_back(v[i+2]);
    sort(ALLOF(w));
    if(w[1]==v[i] || w[1]==v[i+2]){
      ret++;
    }
  }

  cout << ret << endl;
  
  return 0;
}