def main():
    N = int(input())
    A = list(map(int, input().split()))
    count = 0
    for x in range(1, N-1):
        if A[x-1] == A[x+1]:
            continue
        if A[x-1] < A[x] > A[x+1]:
            count += 1
        elif A[x-1] > A[x] < A[x+1]:
            count += 1
    print(count)
            
    

if __name__ == '__main__':
    main()