n=int(input())
s=[int(input()) for i in range(n)]

bool_list = []
for i in range(n -1 ):
    bool = ((s[i] + 1)  == s[i + 1]) or ((s[i] - 1) == s[i + 1])
    bool_list.append(bool)
print((n-1) == bool_list.count(True))