#include <iostream>
#include <string>
#include <cmath>

using namespace std;
#define REP(i,n) for (int i=0;i<(n);i++)
#define SWAP(a,b) (a ^= b,b = a ^ b,a ^= b)

int main()
{
	int N, a[101], b[3], ans = 0;
	cin >> N;
	REP(i, N) cin >> a[i];
	REP(i, N - 2) {
		REP(j, 3) b[j] = a[i + j];
		if (b[0] == b[1] || b[1] == b[2] || b[0] == b[2]) continue;
		if (b[1] > b[2]) SWAP(b[1], b[2]);
		if (b[0] > b[1]) SWAP(b[0], b[1]);
		if (b[1] > b[2]) SWAP(b[1], b[2]);
		if (b[1] != a[i + 1]) ans++;
	}

	cout << ans << endl;
	return 0;
}