#include <stdio.h>



int main(void)

{



int N,i,total=0,A[100]={};

scanf("%d",&N);



for(i=0;i<N;i++){	/*竹の長さをA[i]に格納*/
	scanf("%d",&A[i]);
}

for(i=0;i<(N-2);i++){
	
	if(A[i] != A[i+1] && A[i] != A[i+2] && A[i+1] != A[i+2]){

		if( A[i]<A[i+1] && A[i+1]>A[i+2] ||  A[i]>A[i+1] && A[i+1]<A[i+2] ){
		total++;
		}
	}
}

printf("%d\n",total);






return 0;

}