N = int(input())
A = list(map(int,input().split()))
count = 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 min(A[i],A[i+1],A[i+2]) == A[i+1] or max(A[i],A[i+1],A[i+2]) == A[i+1]:
            count += 1
print(count)