N = int(input())
col = list(map(int,input().split()))
col_3 = []
count = 0
for i in range(len(col)-2):
    col_3.append(col[i:i+3])
for col in col_3:
    if  min(col) < col[0] < max(col):
        count+=1
    elif min(col) < col[2] < max(col):
        count+=1
print(count)