#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
  #include "settings/debug.cpp"
  #define _GLIBCXX_DEBUG
#else
  #define Debug(...) void(0)
#endif
#define rep(i, n) for (int i = 0; i < (n); ++i)
using ll = long long;
using ull = unsigned long long;

int main() {
  int n;
  cin >> n;
  vector<int> a(n);
  rep(i, n) cin >> a[i];
  int ans = 0;
  rep(i, n - 2) {
    vector<int> vec = { a[i], a[i + 1], a[i + 2] };
    sort(vec.begin(), vec.end());
    if (vec[0] == vec[1] || vec[1] == vec[2]) continue;
    if (vec[1] == a[i] || vec[1] == a[i + 2]) ans++;
  }
  cout << ans << endl;
  return 0;
}