#include "bits/stdc++.h"
using namespace std;

int check(int a, int b, int c){
	if (a == b || a == c || b == c) return 0;
	if (max(a, max(b, c)) + min(a, min(b, c)) != a + c) return 1;
	return 0;
}

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

	int ans = 0;
	for (int i = 0; i < N - 2; i++)
	{
		ans += check(A[i], A[i + 1], A[i + 2]);
	}
	cout << ans << endl;
}