#include <bits/stdc++.h>
#define rep(i, ss, ee) for (int i = ss; i < ee; ++i)
using namespace std;

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int N, ans = 0;
  cin >> N;
  vector<int> v(N), tmp(3);
  rep(i, 0, N) { cin >> v[i]; }
  rep(i, 0, N - 2) {
    if (v[i] != v[i + 1] && v[i] != v[i + 2] && v[i + 1] != v[i + 2]) {
      tmp[0] = v[i], tmp[1] = v[i + 1], tmp[2] = v[i + 2];
      sort(tmp.begin(), tmp.end());
      if (tmp[1] != v[i + 1]) ans++;
    }
  }
  cout << ans << endl;
  getchar();
}