#include <sstream>
#include <string>
#include <vector>
#include <map>
#include <algorithm>
#include <iostream>
#include <utility>
#include <set>
#include <cctype>
#include <queue>
#include <stack>
#include <cstdio>
#include <cstdlib>
#include <cmath>

#define INF 1000000000

using namespace std;
typedef long long ll;

const int MAXN = 101;
int A[MAXN];

int main(void) {
    int N;
    cin >> N;
    for (int i = 0; i < N; i++) {
        cin >> A[i];
    }
    int ans = 0;
    for (int i = 0; i < N-2; i++) {
        if (A[i] == A[i+1] || A[i+1] == A[i+2] || A[i+2] == A[i]) continue;
        vector<pair<int, int> > P;
        for (int j = 0; j < 3; j++) {
            P.push_back(make_pair(A[i+j], j));
        }
        sort(P.begin(), P.end());
        if (P[1].second != 1) ans++;
    }
    cout << ans << endl;
    return 0;
}