#include <iostream>
#include <algorithm>
using namespace std;

int main()
{
	int N;
	int A[10 * 10];
	int temp[3];
	int answer = 0;

	cin >> N;
	for(int i = 0; i < N; ++i)
	{
		cin >> A[i];
	}

	for(int j = 0; j < N - 2; ++j)
	{
		for(int i = 0; i < 3; ++i)
		{
			temp[i] = A[j + i];
		}
		sort(temp, temp + 3);
		answer += ((temp[0] != temp[1]) && (temp[1] != temp[2]) && (temp[1] != A[j + 1]));
	}

	cout << answer << endl;
	return 0;
}