#define _GLIBCXX_DEBUG
#include <bits/stdc++.h>
using namespace std;

int main() {
  int N; cin >> N;
  vector<int> X(N);
  for(int i = 0; i < N; i++) cin >> X[i];
  int ans = 0;
  for(int i = 0; i < N-2; i++){
    if(X[i] != X[i+1] and X[i] != X[i+2] and X[i+1] != X[i+2]){
      vector<int> Y(3);
      for(int j = 0; j < 3; j++){
        Y[j] = X[i+j];
      }
      sort(Y.begin(), Y.end());
      if(Y[1] != X[i+1]) ans++;
    }
  }
  cout << ans << endl;    
}