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