#!/usr/bin/env python3

N = int(input())

needed_friends = [0 for i in range(N)]

for _ in range(N):
    prerequisite = int(input())

    if prerequisite >= N:
        # this friend never come
        continue

    needed_friends[prerequisite] += 1

friends_on_party = 0

for i in range(N):
    if i <= friends_on_party:
        friends_on_party += needed_friends[i]
    else:
        break

print(friends_on_party)