#include<stdio.h>
main(){
    int N;
    int A[100];
    scanf("%d",&N);
    for(int i = 0;i < N;i++){
        scanf("%d",&A[i]);
    }
    int count = 0;
    for(int i = 1;i < N-1;i++){
        if(A[i-1]==A[i]||A[i]==A[i+1]||A[i+1]==A[i-1]){continue;}
        if((A[i]>A[i+1]&&A[i]>A[i-1])||
            (A[i]<A[i+1]&&A[i]<A[i-1])){count++;}
    }
    printf("%d\n",count);
}