from collections import Counter
n=input()
inp=Counter(map(int,raw_input().split()))
maxi=0
k = sorted(inp.keys())
for i in range(len(k)-1):
    if k[i+1]-k[i] == 1 and (inp[k[i+1]]+inp[k[i]])>maxi:
        maxi = inp[k[i+1]]+inp[k[i]]
    if inp[k[i]] > maxi:
        maxi = inp[k[i]]
if inp[k[-1]] > maxi:
    maxi = inp[k[-1]]
if maxi <= 2:
    print 2
else:
    print maxi