import sys
readline=sys.stdin.readline

N=int(readline())
A=list(map(int,readline().split()))
ans=0
for i in range(N-2):
    a,b,c=A[i:i+3]
    if len({a,b,c})==3 and sorted([a,b,c])[1] in (a,c):
        ans+=1
print(ans)