N = int(input())

A = []
for _ in range(N):
    A.append(int(input()))
    
A.sort(reverse=True)

L = []
for a in A:
    for i in range(len(L)):
        if L[i] > a + 1:
            L[i] = a
            break
    else:
        L.append(a)

print(len(L))