#include <bits/stdc++.h>
#define rep(i,n) for(int i=0; i<(n); i++)
using namespace std;
typedef long long ll;

int main(){
	int n; cin>>n;
	vector<int> v(n);
	rep(i,n) cin>>v[i];
	int ans=0;
	rep(i,n-2){
		if(v[i]>v[i+1]&&v[i+1]<v[i+2]&&v[i]!=v[i+2]) ans++;
		else if(v[i]<v[i+1]&&v[i+1]>v[i+2]&&v[i]!=v[i+2]) ans++;
	}
	cout << ans << endl;
}