#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
#define rep(i, n) for(int i = 0; i < (n); i++)
#define rep1(i, n) for(int i = 1; i <= (n); i++)
#define co(x) cout << (x) << "\n"
#define cosp(x) cout << (x) << " "
#define ce(x) cerr << (x) << "\n"
#define cesp(x) cerr << (x) << " "
#define pb push_back
#define mp make_pair
#define Would
#define you
#define please


int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);


	int N;
	cin >> N;
	int A[100];
	rep(i, N) cin >> A[i];

	int kotae = 0;
	rep(i, N - 2) {
		int x, y, z;
		x = A[i];
		y = A[i + 1];
		z = A[i + 2];
		if (x != y && y != z && z != x) {
			if ((z - x) * (y - x) < 0 || (x - z) * (y - z) < 0) {
				kotae++;
			}
		}
	}
	co(kotae);

	Would you please return 0;
}