#define _USE_MATH_DEFINES
#include <algorithm>
#include <cstdio>
#include <functional>
#include <iostream>
#include <cfloat>
#include <climits>
#include <cstring>
#include <cmath>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <time.h>
#include <vector>
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> i_i;
typedef pair<ll, int> ll_i;
typedef pair<double, int> d_i;
typedef pair<ll, ll> ll_ll;
typedef pair<double, double> d_d;
struct edge { int u, v; ll w; };

ll MOD = 1000000007;
ll _MOD = 1000000009;
double EPS = 1e-10;

int main() {
	int N; cin >> N;
	vector<int> A(N);
	for (int i = 0; i < N; i++)
		cin >> A[i];
	int cnt = 0;
	for (int i = 0; i + 2 < N; i++) {
		vector<int> a(3);
		a[0] = A[i]; a[1] = A[i + 1]; a[2] = A[i + 2];
		sort(a.begin(), a.end());
		a.erase(unique(a.begin(), a.end()), a.end());
		if (a.size() < 3) continue;
		if (A[i] == a[1] || A[i + 2] == a[1]) cnt++;
	}
	cout << cnt << endl;
}