using System;
using System.Linq;

class No116{
    static void Main(){
        var a=Int32.Parse(Console.ReadLine())-1;
        var b=Console.ReadLine().Split(' ').Select(x=>Int32.Parse(x)).ToArray();
        var c=0;
        for(int i=1;i<a;++i){
            if(b[i-1]==b[i+1])continue;
            if(b[i]-b[i-1]>0&&b[i]-b[i+1]>0)c++;
            else if(b[i]-b[i-1]<0&&b[i]-b[i+1]<0)c++;
        }
        Console.WriteLine(c);
    }
}