#include <climits>
#include <iostream>
#include <math.h>
#include <algorithm>
#include <map>
#include <vector>
#include <numeric>
using namespace std;

#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rep2(i, d, n) for (int i = d; i < (int)(n); i++)
#define rRep(i,n) for(int i = n; i >= 0; i--)
#define rRep2(i,d,n) for(int i = n; i >= d; i--)
#define forEach(arr) for (auto& it:arr)


int main(void){
    
    int n,c = 0;
    cin >> n;
    int bamboo[n];
    rep(i,n){
        cin >> bamboo[i];
    }
    rep2(i,1,n - 1){
        int l = bamboo[i-1],m = bamboo[i],r = bamboo[i+1];
        int mi = min({l,m,r});
        int ma = max({l,m,r});
        if(l == m || l == r || m == r)continue;
        if(mi == m || m == ma) {
            c++;
        } else continue;
    }
    cout << c;
}