#include <iostream>
#include <vector>
#include <map>
#include <array>

using namespace std;
template<class T> bool maxPointer(T& a, T b) { if (a < b) {a = b;return true;}return false;}
template<class T> int maxReturn(T a, T b) {if (a > b) {return a;} else {return b;}}
template<class T> bool minPointer(T& a, T b) {if (a < b) {a = b;return true;}return false;}

bool matuCheck(int a[], int i) {
	if ((a[i] > a[i-1] && a[i] > a[i+1]) || (a[i] < a[i-1] && a[i] < a[i+1])) {
		if (a[i] != a[i-1] && a[i] != a[i+1] && a[i-1] != a[i+1]) {return true;}
	}
	
	return false;
}

int main(void){
    int N;
    cin >> N;
    int matu[N];
    int res = 0;
    
    for (int i = 0; i < N; i++) {
        cin >> matu[i];
    }

    for (int i = 1; i < N -1; i++) {
        if (matuCheck(matu, i)) res++;
    }

    cout << res << endl;
}