#include<iostream>
#include<vector>
#include<algorithm>

using namespace std;

typedef long long ll;

const ll mod = 1000000007;

bool func(int a , int b , int c){
    if(a == b || b == c || c == a){
        return false;
    }
    if((b < a && b < c) || (b > a && b > c)){
        return true;
    }
    return false;
}

int main(void){
    int n;
    cin >> n;
    vector<int> a(n);
    for(int i = 0; i < n; i++){
        cin >> a[i];
    }
    int ans = 0;
    for(int i = 2; i < n; i++){
        ans += func(a[i - 2] , a[i - 1] , a[i]);
    }
    cout << ans << endl;
    return 0;
}