#include <vector>
#include <queue>
#include <deque>
#include <stack>
#include <algorithm>
#include <functional>
#include <iostream>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <ctime>
#include <cctype>
#include <string>

using namespace std;

bool is_kadomatu(int a, int b, int c) {
  if (a == b || b == c || a == c) return false;
  int t[] = {a, b, c};
  sort(t, t+3);
  if (t[1] == a || t[1] == c) return true;
  return false;
}

int main() {
  int n;
  cin >> n;
  int a[n];
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }
  int cnt = 0;
  for (int i = 0; i < n-2; i++) {
    cnt += is_kadomatu(a[i], a[i+1], a[i+2]);
  }
  std::cout << cnt << std::endl;
}