#include <iostream>
#include <algorithm>
#include <vector>
#include <string>
using namespace std;
typedef long long LL;

int main(){
    LL N;
    cin >> N;
    vector<int> height(N);
    for(int i = 0; i < N; i++){
        cin >> height[i];
    }
    int cnt = 0;
    for(int i = 0; i+2 < N; i++){
        if(height[i] == height[i+1] || height[i+1] == height[i+2]) continue;
        if((height[i] < max(height[i+1],height[i+2]) && height[i] > min(height[i+1], height[i+2]))
            || (height[i+2] < max(height[i], height[i+1]) && height[i+2] > min(height[i], height[i+1]))) cnt++;
    }
    cout << cnt << endl;
    return 0;
}