#include <algorithm>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <string>
#include <vector>
using namespace std;
typedef pair<int, int> pii;
#define rep(i,x) for(int i=0;i<(int)(x);++i)

int main(){
    int n, cnt = 0, a[100] = {};
    cin >> n;
    rep(i, n) cin >> a[i];
    rep(i, n-2) {
        if (a[i] < a[i+1] && a[i+2] < a[i+1] && a[i] != a[i+2]) ++cnt;
        if (a[i+1] < a[i] && a[i+1] < a[i+2] && a[i] != a[i+2]) ++cnt;
    }
    cout << cnt << endl;
    return 0;
}