#include<iostream>
#include<string>
#include<string.h>
#include<algorithm>
#include<stdio.h>
#include<cmath>
#include<vector>
#include<utility>
#include<stack>
#include<queue>
#include<list>

#define FOR(i, a, b) for(int i=(a);i<=(b);i++)
#define RFOR(i, a, b) for(int i=(a);i>=(b);i--)
#define MOD 1000000007
#define INF 1000000000

using namespace std;


int main(void) {
	int n;
	int count = 0;
	int a[101];

	cin >> n;
	FOR(i, 1, n) {
		cin >> a[i];
	}
	FOR(i, 1, n - 2) {
		if (((a[i + 2] > a[i + 1] && a[i + 1] < a[i]) || (a[i + 2] < a[i + 1] && a[i + 1] > a[i]))&&(a[i+2]!=a[i])) {
			count++;
		}
	}
	cout << count << endl;
	return 0;
}