#define _GLIBCXX_DEBUG
#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (int)n; i++)
#define repp(i, m, n) for (int i = (int)m; i < (int)n; i++)
#define all(v) begin(v), end(v)
using namespace std;
using ll = long long int;

int main() {
    // 入力
    int N, A[100];
    cin >> N;
    rep(i, N) cin >> A[i];

    // 門松列のカウント
    int cnt = 0;
    rep(i, N - 2) {
        int B[3] = {A[i], A[i + 1], A[i + 2]};
        sort(begin(B), end(B));
        if (B[0] != B[1] && B[1] != B[2] && B[1] != A[i + 1]) cnt++;
    }

    // 出力
    cout << cnt << endl;
}