#include <algorithm>
#include <iostream>
#include <iomanip>
#include <map>
#include <numeric>
#include <set>
#include <sstream>
#include <string>
#include <vector>
#include <cmath>
using namespace std;

#define FOR(i,s,e) for (int i = int(s); i != int(e); i++)
#define FORIT(i,c) for (typeof((c).begin()) i = (c).begin(); i != (c).end(); i++)
#define ISEQ(c) (c).begin(), (c).end()

int main(){
	int  N;
	cin >> N;
	int A[N];
	FOR(i,0,N) cin >> A[i];
	int ans = 0;
	FOR(i,1,N-1){
		if(A[i-1] ==  A[i] || A[i] == A[i+1] || A[i-1] == A[i+1]) continue;
		if(max(A[i-1],A[i+1]) > A[i] && min(A[i-1],A[i+1]) < A[i]) continue;
		ans++;
	}
	cout << ans  << endl;
	return 0;
}