import kotlin.math.*

fun main()
{
    val N = readLine()!!.toInt()
    val A = readLine()!!.split(" ").map(String::toInt)

    var ans = 0
    for (i in 0..N-3) {
        if (isDistinct(A[i], A[i+1], A[i+2]) && A[i+1] != med(A[i], A[i+1], A[i+2])) {
            ans++;
        }
    }
    
    println(ans)
}

fun isDistinct(a: Int, b:Int, c:Int): Boolean {
    return a != b && b != c && c != a
}

fun med(a: Int, b: Int, c: Int): Int {
    if (a <= b) {
        if (c <= a) return a
        if (c <= b) return c
    } else {
        if (a <= c) return a
        if (b <= c) return c        
    }
    return b
}