实在想不出还有啥样本过不了

n = input()
l = list(map(int, input().split()))
l.append(0)

max = 1
count = 0
for i in range(len(l) - 1):
  if l[i] == l[i + 1] - 1:
    count += 1
  else:
    if count > max:
      max = count 
    count = 0  
print(max + 1)