#include<bits/stdc++.h>
using namespace std;
using ll = long long;

template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }

#define FOR(i,a,b) for(ll i=(a);i<(b);++i)
#define ALL(v) (v).begin(), (v).end()
#define p(s) cout<<(s)<<endl
#define p2(s, t) cout << (s) << " " << (t) << endl
#define br() p("")
#define pn(s) cout << (#s) << " " << (s) << endl
#define p_yes() p("Yes")
#define p_no() p("No")

const ll mod = 1e9 + 7;
const ll inf = 1e18;

void vprint(vector<ll> A){
    ll L = A.size();
    FOR(i, 0, L){
        if(i) cout << ' ';
        cout << A[i];
    }
    cout << endl;
}

bool is_kadomatsu(ll a, ll b, ll c){
    if(a==b || b==c || a==c){
        return false;
    }
    ll diff0 = b-a;
    ll diff1 = c-b;
    if(diff0*diff1<0){
        return true;
    }else{
        return false;
    }
}

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);

    // input
    ll N;
    cin >> N;

    vector<ll> A(N);
    FOR(i, 0, N){
        cin >> A.at(i);
    }

    ll cnt = 0;
    FOR(i, 0, N-2){
        if(is_kadomatsu(A[i], A[i+1], A[i+2])){
            cnt++;
        }
    }
    p(cnt);
    
    return 0;
}