#include <bits/stdc++.h>

using namespace std;

int main() {
	int N; cin >> N;
	vector<int> v(N);
	for (int i = 0; i < N; ++i) {
		cin >> v[i];
	}
	int ans = 0;
	for (int i = 0; i < N-2; ++i) {
		set<int> s;
		for (int j = 0; j < 3; ++j) {
			s.insert(v[i+j]);
		}
		auto it = s.begin(); ++it;
		if (s.size() == 3 && *it != v[i+1]) ++ans;
	}
	cout << ans << endl;
}