N = int(input())
A = list(map(int,input().split()))
answer = 0
for i in range(N-2) :
    if (A[i] != A[i+1]) and (A[i+1] != A[i+2]) and (A[i+2] != A[i]) :
        if ((A[i] < A[i+1]) and (A[i+1] > A[i+2])) or ((A[i] > A[i+1]) and (A[i+1] < A[i+2])) :
            answer += 1
print(answer)