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

int main(void) {
	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) {
		set<int> s(A.begin() + i, A.begin() + i + 3);
		if(s.size() == 3 and *next(s.begin()) != A[i + 1]) ans += 1;
	}
	
	cout << ans << "\n";
}