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

int main(){
	int n;
	cin >> n;
	vector<int> data(n);
	for(auto &a:data) cin >> a;
	int count = 0;
	for(int i = 0; i < n - 2; i++){
		int x = data[i], y = data[i+1], z = data[i+2];
		//		cout << x << " " << y << " " << z << endl;
		if(x != y and y != z and x != z){
			if(max(x,max(y,z)) == y) count++;
			if(min(x,min(y,z)) == y) count++;
		}
	}
	cout << count << endl;
	return 0;
}