#include <bits/stdc++.h>
#define For(i, a, b) for(int i = a; i < b; i++)
#define rep(i, n) For(i, 0, n)
#define rFor(i, a, b) for(int i = a; i >= b; i--)
#define ALL(v) (v).begin(), (v).end()
#define rALL(v) (v).rbegin(), (v).rend()
using namespace std;

using lint = long long;
using ld = long double;

int INF = 2000000000;
lint LINF = 1000000000000000000;

int main() {
    int n;
    cin >> n;
    vector<int> a(n);
    rep(i, n) {
        cin >> a[i];
    }
    auto ok = [](int x, int y, int z) -> bool {
        vector<int> v = {x, y, z};
        sort(ALL(v));
        if (v[0] == v[1] || v[1] == v[2]) {
            return false;
        }
        if (x == v[1] || z == v[1]) {
            return true;
        } else {
            return false;
        }
    };
    int ans = 0;
    rep(i, n) {
        if (i + 2 < n) {
            if (ok(a[i], a[i + 1], a[i + 2])) {
                ans++;
            }
        }
    }
    cout << ans << endl;
}