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

int main() {
    int N, A[1000];
    cin >> N;
    for (int i = 0; i < N; i++) {
        cin >> A[i];
    }

    auto ok = [&](int a, int b, int c) {
        vector<int> V{a, b, c};
        sort(V.begin(), V.end());
        return V[0] != V[1] && V[1] != V[2] && (V[1] == a || V[1] == c);
    };

    int ans = 0;
    for (int i = 0; i < N - 2; i++) {
        if (ok(A[i], A[i + 1], A[i + 2]))
            ans++;
    }
    cout << ans << endl;
}