#define _USE_MATH_DEFINES
#include  <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include <string>
#include<math.h>
#include<iomanip>
#include<stdio.h>
#include <stdlib.h>
#include<stdio.h>
#include <queue>
#include<map>
#include <fstream>
#include <sstream>
#include<set>
#include<stack>
#include<time.h>
#include <sys/types.h>
#include <tuple>
#include <numeric>
#include <iterator>

typedef long long int ll;
//#include<bits/stdc++.h>


using namespace std;





int main()
{

    int Q;
    Q = 1;
    //cin >> Q;


    while (Q--) {
        int n;
        cin >> n;
        vector<int>p(n);
        for (int i = 0; i < n; i++)cin >> p[i];
        int ans = 0;
        for (int i = 1; i < n - 1; i++) {
            vector<int>t;
            set<int>s;
            t.push_back(p[i - 1]); s.insert(p[i - 1]);
            t.push_back(p[i + 1]); s.insert(p[i + 1]);
            t.push_back(p[i]); s.insert(p[i]);
            sort(t.begin(), t.end());
            if (t[1] == p[i])continue;
            if (s.size() != 3)continue;
            ans++;
        }
        cout << ans;

    }
    return 0;
}