let itemCount = readLine()

let inpt = readLine()!.split(separator: " ").map{Int($0)!}

var ans = 0
for i in 2..<inpt.count {
    if(inpt[i-1] > inpt[i-2] && inpt[i-1] > inpt[i] && inpt[i] != inpt[i-2]) {
        ans += 1
    } else if(inpt[i-1] < inpt[i-2] && inpt[i-1] < inpt[i] && inpt[i] != inpt[i-2]) {
        ans += 1
    }
}
print(ans)