#include<bits/stdc++.h>

using namespace std;

int main(){

	int resp, t, v[100000];

	while(cin>>t && t){

		resp = 0;

		for(int i=1;i<=t;i++) cin>>v[i];

		v[0] = v[t];
		v[t+1] = v[1];

		for(int i=1;i<=t;i++){

			if(v[i-1]>v[i] && v[i+1]>v[i]) resp++;
			if(v[i-1]<v[i] && v[i+1]<v[i]) resp++;

		}

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

	}

	return 0;
}