#include <algorithm>
#include <iostream>

using namespace std;

int main(int argc, const char *argv[])
{
	int N;
	cin >> N;
	int A[N];
	for (auto &&a : A) cin >> a;
	int cnt = 0;
	for (int i = 0; i < N - 2; i++) {
		int abc[] = {A[i], A[i + 1], A[i + 2]};
		sort(abc, &abc[3]);
		if (abc[1] == A[i] || abc[1] == A[i + 2])
			if (A[i] != A[i + 1] && A[i] != A[i + 2] && A[i + 1] != A[i + 2]) cnt++;
	}
	cout << cnt << endl;
	return 0;
}