#include <iostream>
#include <math.h>
#include <algorithm>
#include <functional>
#include <vector>
#include <typeinfo>
#include <climits>
#include <ctype.h>
#include <string>
#include <stdio.h>
using namespace std;
#define INIT cin.tie(0); ios::sync_with_stdio(false);
#define FOR(i, a, n) for (int i = a; i < n; i++)
#define REP(i, n) for(int i = 0; i < n; i++)

int main()
{
    INIT;

    int n;
    cin >> n;
    int A[n];
    REP(i, n) cin >> A[i];

    int cnt = 0;
    REP(i, n - 2){
        int mi = INT_MAX;
        if(A[i] != A[i + 1] and A[i + 1] != A[i + 2] and A[i] != A[i + 2]){
            int B[3];
            B[0] = A[i];
            B[1] = A[i + 1];
            B[2] = A[i + 2];
            sort(B, B + 3);
            if(A[i] == B[1] or A[i + 2] == B[1]) cnt++;
        }
    }
    cout << cnt << endl;

    return 0;
}