#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <string>
#include <cmath>
#include <algorithm>
#include <vector>
#include <queue>
#include <stack>
#include <map>
#include <set>
#include <cstring>

typedef long long ll;
using namespace std;

#define mod 1000000009
#define INF 1000000000
#define LLINF 2000000000000000000LL
#define PI 3.1415926536

#define SIZE 1024

int main(){
    int n,a[100],ans=0,s[3];
    
    scanf("%d",&n);
    
    for(int i=0;i<n;i++) scanf("%d",&a[i]);
    
    for(int i=1;i<n-1;i++){
        
        for(int j=-1;j<=1;j++)
            s[j+1] = a[i+j];
        
        sort(s,s+3);
        
        if(s[0]==s[1] || s[1]==s[2]) continue;
        
        if(s[1]==a[i-1] || s[1]==a[i+1]) ans++;
        
    }
    
    printf("%d\n",ans);
    
    return 0;
}