#include "bits/stdc++.h"
using namespace std;
#define ll long long
#define endl '\n'
#define rep(i, n) for(int i = 0; i < (int)(n); i++)
#define rep2(i,a,b) for(int i=int(a);i<int(b);i++)
#define pb emplace_back
#define all(x) (x).begin(),(x).end()
ll mod=1e9+7;
ll mod2=998244353;
const int INF=1e9;


 
signed main() {
	ll n;
	cin>>n;
	vector<ll> a(n);
	rep(i,n)cin>>a[i];
	ll ans=0;
	rep(i,n-2){
		if((a[i]<a[i+1]&&a[i+2]>a[i+1])||(a[i]>a[i+1]&&a[i+2]<a[i+1]))ans++;
	}
	cout<<ans<<endl;
}

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