N=int(input())
A=list(map(int,input().split()))
tmp=[]
cnt=0
for i in range(N-2):
    a=A[i]
    b=A[i+1]
    c=A[i+2]
    tmp=[a,b,c]
    tmp.sort(reverse=True)
    if (tmp[1]==a or tmp[1]==c) and (a!=b and b!=c and c!=a):
        cnt+=1

print(cnt)