from heapq import heappush, heapreplace

n=int(input())
A=[]
for i in range(n):
    a=int(input())
    A.append(a)

A.sort()
q=[A[0]]
for a in A[1:]:
    if a<=q[0] + 1:
        heappush(q,a)
    else:
        heapreplace(q,a)

print(len(q))