module main;

import std;

void main()
{
	// 入力
	int N = readln.chomp.to!int;
	auto A = readln.split.to!(int[]);
	// 答えの計算
	int ans = 0;
	foreach (i; 0 .. N - 2) {
		auto tmp = A[i .. i + 3].dup;
		if (tmp[0] == tmp[1] || tmp[1] == tmp[2] || tmp[2] == tmp[0])
			continue;
		auto e = tmp.enumerate.array;
		e.sort!"a[1] < b[1]";
		ans += e[1][0] == 0 || e[1][0] == 2;
	}
	// 答えの出力
	writeln(ans);
}