n=int(input())
l=[]
s=input()
l=s.split()
l=list(map(int,l))
l1=[]
m=1
for i in range(0,n-1):
    j=i+1
    if l[i]==l[j]-1:
        m+=1
    else:
        l1.append(m)
        m=1
l1.sort()
print(l1[-1])