/**
                    Unmesh Kumar
     Indian Institute of Technology,Roorkee CSE'22
""If everything seems under control then you are not going fast enough!""

**/



#include <iostream>
#include <bits/stdc++.h>
#include <sstream>
#include <cmath>
#include <algorithm>
#include <map>
#include <iterator>
#include <iomanip>
#include <queue>
#include <utility>

using namespace std;
#define ll long long int
#define c(x) ll x;cin>>x
#define cc(x,y) ll x,y;cin>>x>>y
#define ccc(x,y,z) ll x,y,z; cin>>x>>y>>z
#define db long double
#define fast cin.tie(NULL); cout.tie(NULL); ios_base::sync_with_stdio(false)
#define inf 200000000000000
#define mod 1000000007
#define eps 1e-7
#define PI 3.1415926535897932385
#define pb push_back
#define bitc  __builtin_popcountll
#define mp make_pair
#define ff first
#define ss second
#define all(ar) ar.begin(), ar.end()
#define len(x) (ll)(x).size()
#define fr(i,a,b) for (ll i = (a), _b = (b); i <= _b; i++)
#define rep(i,n) for (ll i = 0, _n = (n); i < _n; i++)
#define repr(i,n) for (ll i = n - 1; i >= 0; i--)
#define frr(i,a,b) for (ll i = (a), _b = (b); i >= _b; i--)
#define foreach(it,ar) for (auto it = ar.begin(); it != ar.end(); it++)
#define fill(ar,val) memset(ar, val, sizeof(ar))
#define print(arr,n) rep(i,n) cout<<arr[i]<<" "; cout<<endl
#define print2(arr,n,m) rep(i,n){ rep(j,m) cout<<arr[i][j]<<" "; cout<<endl; }
#define dyn1(arr,n) ll* arr=new ll[n]; rep(i,n) arr[i]=0;
#define dyn2(arr,n,m) ll** arr=new ll*[(n)]; rep(i,n) arr[i]=new ll[m]; rep(i,n) rep(j,m) arr[i][j]=0
#define carr(arr,n) rep(i,n) cin>>arr[i]
#define carr2(arr,n,m) rep(i,n) rep(j,m) cin>>arr[i][j]
#define debug(a) cout<<" :: "<<#a<<" == "<<a<<endl
#define debug2(a,b) cout<<" :: "<<#a<<" == "<<a<<" :: "<<#b<<" == "<<b<<endl
typedef pair<int,int> ii;
typedef vector<ii> vii;

inline ll maxim(ll a,ll b) {if(a>b) return a; else return b;}
inline ll minim(ll a,ll b) {if(a<b) return a; else return b;}
inline bool equals(double a, double b){ return fabs(a - b) < 1e-9; }
ll gcd(ll a, ll b){ return b==0 ? a : gcd(b, a%b); }


ll binpow(ll a, ll b)
{
    ll res = 1;
    while (b > 0)
    {
        if (b & 1) res = (res * a)%mod;
        a = (a * a)%mod;
        b >>= 1;
    }
    return res;
}

/*
ll ncr(ll n,ll r)
{
  if(r>n) return 0;
  if((n==r)||(r==0))
  {
    return 1;
  }
  if(r==1) return n;
  if(r>n-r)
  {
    r=n-r;
  }

  string key=to_string(n)+"|"+to_string(r);
  if(m.find(key)==m.end())
  {
    ll answer=ncr(n-1,r-1)+ncr(n-1,r);
    m[key]=answer;
  }
  //cout<<key<<" "<<m[key]<<endl;
  return m[key];
}
*/

int modInverse(int a, int m)
{
    int m0 = m;
    int y = 0, x = 1;
    if (m == 1)
      return 0;
    while (a > 1)
    {
        // q is quotient
        int q = a / m;
        int t = m;
        // m is remainder now, process same as
        // Euclid's algo
        m = a % m, a = t;
        t = y;
        // Update y and x
        y = x - q * y;
        x = t;
    }
    // Make x positive
    if (x < 0)
       x += m0;
    return x;
}


class node
{
public:
  ll w,h,index;
  node()
  {
    this->w=0;
    this->h=0;
    this->index=0;
  }
  node(ll w,ll h,ll index)
  {
    this->w=w;
    this->h=h;
    this->index=index;
  }
};

bool comp(node a,node b)
{
  if(a.w<b.w) return true;
  else if(a.w>b.w) return false;
  else
  {
    return a.h<b.h;
  }
}

ll arr[200001];
ll dp[100001];
ll y[100001];

int main()
{
  fast;
  //freopen("input.txt", "r", stdin);
  //freopen("output.txt", "w", stdout);
  cout<<setprecision(10)<<fixed;
  c(n);
  carr(arr,n);
  ll count=0;
  fr(o,1,n-2)
  {
    if((arr[o]<arr[o+1]&&arr[o]>arr[o-1])||(arr[o]>arr[o+1]&&arr[o]<arr[o-1]))
    count++;
  }
  cout<<count<<endl;
  return 0;
}


Measured memory_bytes: 256