#include <cstdio>
#include <iostream>
#include <sstream>
#include <fstream>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <string>
#include <vector>
#include <list>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <bitset>
#include <numeric>
#include <limits>
#include <climits>
#include <cfloat>
#include <functional>
using namespace std;

int main()
{
    int n;
    cin >> n;
    vector<int> a(n);
    for(int i=0; i<n; ++i)
        cin >> a[i];

    int ret = 0;
    for(int i=1; i<n-1; ++i){
        if(a[i-1] != a[i+1] && ((a[i-1] < a[i] && a[i] > a[i+1]) || (a[i-1] > a[i] && a[i] < a[i+1])))
            ++ ret;
    }
    cout << ret << endl;

    return 0;
}