#include <iostream>
#include <vector>
#include <string>
#include <cstring>
#include <math.h>
#include <cmath>
#include <limits.h>
#include <map>
#include <set>
#include <queue>
#include <algorithm>
#include <functional>
#include <stdio.h>
using namespace std;

long long MOD = 1000000007;

int main() {
    
    int ans = 0;
    int N;
    cin >> N;
    vector<int> A(N);
    for ( int i = 0; i < N; i++ ) {
        cin >> A[i];
    }
    
    int a1,a2,a3;
    a1 = A[0];
    a2 = A[1];
    for ( int i = 2; i < N; i++ ) {
        if ( a1 == a2 || a1 == A[i] || a2 == A[i] )  { 
            a1 = a2;
            a2 = A[i];
            continue;
       }
        if ( !( ( a1 < a2 && a2 < A[i] ) || ( A[i] < a2 && a2 < a1 ) ) ) { ans++; }
        a1 = a2;
        a2 = A[i];
    }
    
    cout << ans << endl;
    
    return 0;
}