n = int(input())
a = list(map(int, input().split()))

count = 0
for i in range (1, len(a)-1):
    c = a[i] - a[i + 1]
    if ( c > 0) :
        if((a[i] - a[i-1])>0):
            count = count+1

    elif( c<0 ):
         if((a[i]-a[i-1])<0):
             count = count+1
print(count)

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