#include<iostream>
#include<vector>
#include<algorithm>

int main() {
	int n;
	std::vector<int> v;

	std::cin >> n;
	for (int i = 0; i < n; i++) {
		int num;
		std::cin >> num;
		v.push_back(num);
	}

	int count = 0;
	std::vector<int> v2(3);
	for (int i = 0; i < n - 2; i++) {
		if (v[i] != v[i + 1] && v[i + 1] != v[i + 2] && v[i] != v[i + 2]) {
			v2[0] = v[i];
			v2[1] = v[i + 1];
			v2[2] = v[i + 2];
			std::sort(v2.begin(), v2.end());
			if (v2[1] != v[i + 1]) {
				count++;
			}
		}
	}

	std::cout << count << std::endl;
	return 0;
}