#include <vector>
#include <string>
#include <algorithm>
#include <numeric>
#include <iostream>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <utility>
#include <list>
#include <functional>
#include <queue>
#include <map>
#include <iterator>
#include <iomanip>
#include <complex>
using namespace std;

using ull = unsigned long long;
using ll = long long;


int main()
{
  int n;
  cin >> n;
  vector<int> data(n);

  for(auto&& x : data)  cin >> x;

  int count = 0;
  for(int i = 0; i+2 < data.size(); ++i)
  {
    vector<int> d(3);
    d.at(0) = data.at(i);
    if(d.at(0) != data.at(i+1))
      d.at(1) = data.at(i +1);
    else
      continue;

    if(d.at(0) != data.at(i+2) && d.at(1) != data.at(i+2))
      d.at(2) = data.at(i+2);
    else
      continue;

    sort(d.begin(),d.end());
    int tmp = d.at(1);

    if((tmp == data.at(i) || tmp == data.at(i+2)))
      ++count;
  }

  cout << count << endl;
}