#include <bits/stdc++.h>
using namespace std;

int main()
{
int i;
int N;
int A[ 110 ];
int iSum;
int a;
int b;
int c;

	cin >> N;
	for( i = 0; i < N; i++ ) cin >> A[ i ];

	iSum = 0;
	a = A[ 0 ];
	b = A[ 1 ];
	for( i = 2; i < N; i++ )
	{
		c = A[ i ];
		if( a != c )
		{
			if( b < a && b < c )
				iSum++;
			else if( b > a && b > c )
				iSum++;
		}

		a = b;
		b = c;
	}

	cout << iSum << endl;

	return 0;
}