#include <iostream>
#include <vector>
#include <algorithm>
#include <string>

#define REP(i, a, b) for (int i = int(a); i < int(b); i++)

using namespace std;

typedef long long int lli;

int main() {
    int N;
    cin >> N;
    vector<int> v(N);
    REP (i, 0, N) cin >> v[i];
    int ans = 0;
    REP (i, 0, N - 2) {
        int a = v[i];
        int b = v[i + 1];
        int c = v[i + 2];
        if (a != b && b != c && c != a) {
            if (a > b) swap(a, b);
            if (b > c) swap(b, c);
            if (a > b) swap(a, b);
            if (v[i] == b || v[i + 2] == b) ans++;
        }
    }
    cout << ans << endl;
    return 0;
}