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#include<iostream>
using namespace std;

int main()
{
	ios_base::sync_with_stdio(0);
  cin.tie(0);
	int n, ile = 0;
	cin >> n;
	int* tab = new int[n];
	for(int i = 0; i < n; i++)
	{
		cin >> tab[i];
	}
	for(int i = 1; i < n; i=i+2)
	{
		if((tab[i-1] > tab[i] and tab[i] < tab[i+1]) or (tab[i-1] < tab[i] and tab[i] > tab[i+1]))
		{
			
		}
		else
		{
			ile++;
		}
	}
	cout << ile << endl;
	
	return 0;
}