#include<bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp> 
using namespace std;
using namespace __gnu_pbds;
#define fio ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL)
using ll=long long;
using ld=long double;
#define inf INT_MAX  
const ll infll=1ll*inf*inf;
#define mode 998244353
#define mod 1000000007
const long double PI=3.141592653589793238462643383279502884197169399375105820974944;
#define pb push_back
#define mp make_pair
#define ub upper_bound 
#define lb lower_bound 
#define far(i,a,b) for (ll i = a; i <= b; i++)
#define rev(i,a,b) for (ll i = a; i >= b; i--)
#define all(v) v.begin(),v.end()
#define ff first
#define ss second
#define sz(a) int((a).size())
#define ftr(c,i) for(auto i = begin(c); i != end(c); i++)
#define rtr(c,i) for(auto i = rbegin(c); i != rend(c); i++)
#define pre(c,x) ((c).find(x) != (c).end())
#define cpre(c,x) (find(all(c),x) != (c).end())
#define mii map<int,int>
#define mll map<ll,ll>
#define vs vector<string>
#define vi vector<int>
#define vl vector<ll>
#define pll pair<ll,ll>
#define pii pair<int,int>
#define vii vector<pii >
#define vll vector<pll >
#define vvl vector<vector<ll> >
#define vvi vector< vector<int > >
#define wh(t) while(t --)
#define print(arr) for (auto it = arr.begin(); it != arr.end(); ++it) cout <<(ll)(it-arr.begin()) << ')' <<  *it << ' '; cout << endl;
#define printii(arr) for (auto it = arr.begin(); it != arr.end(); ++it) cout << it->ff<<' '<<it->ss << endl; cout << endl;
#define MID(i,j) ((i)+(j))/2
#define nl '\n' 
#define lcm(a,b) ((a)*(b))/gcd((a),(b))
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());

ll max(ll a,ll b) {return (a>b? a:b);}
ll min(ll a,ll b) {return (a>b? b:a);}
ll gcd(ll a,ll b) {return (b==0)? a:gcd(b,a%b); }
ll gcd(ll a,ll b,ll & x,ll & y) {
    if (a == 0) {
        x = 0;
        y = 1;
        return b;
    }
    ll x1, y1;
    ll d = gcd(b % a, a, x1, y1);
    x = y1 - (b / a) * x1;
    y = x1;
    return d;
}
ll modinv(ll a,ll b){
	ll x,y;
	gcd(a, b, x, y);
	return (x%b + b)%b;
}
ll ceiling(ll a,ll b){ 
        if(a%b==0) return a/b;
        else return a/b+1;
}
ll bpow(ll a,ll b,ll m){
	a%=m;
	ll res=1;
	while(b>0){
		if(b&1) res=(res*a)%m;
		b>>=1;
		a=a*a%m;
	}
	return res;
}
vvl mat_mul(vvl mat1,vvl mat2,ll mod1){
        vvl res(sz(mat1),vl(sz(mat2[0]),0));
        far(i,0,sz(mat1)-1){
                far(j,0,sz(mat2[0])-1){
                        far(k,0,sz(mat1[0])-1){
                                res[i][j]=(res[i][j]+(mat1[i][k]*mat2[k][j])%mod1)%mod1;          
                        }   
        }   
        }   
        return res;
}

vvl mat_exp(vvl mat,ll p,ll mod1){ 
        vvl res(sz(mat),vl(sz(mat[0]),0));
        far(i,0,sz(mat)-1) res[i][i]=1;
        while(p>0){
                if(p&1) res=mat_mul(res,mat,mod1);   
                mat=mat_mul(mat,mat,mod1);
                p>>=1;
        }   
        return res;
}
// find_by_order()  // order_of_key()
typedef tree< int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update>
o_s;

const int N=3*1e5+5;
ll n,m,q,t,k,x,y,z,ans;
int main()
{
	fio;
	cin >> n;
	vi a(n+1);
	far(i,1,n) cin >>a[i];
	far(i,2,n-1){
		if(a[i]>a[i-1] && a[i]<a[i+1]) ans++;
		else if(a[i]<a[i-1] && a[i]>a[i+1]) ans++;
	}
	cout << ans <<nl;
}



Measured memory_bytes: 256