import sys
from collections import defaultdict

def main():
    input = sys.stdin.read().split()
    n = int(input[0])
    A = list(map(int, input[1:n+1]))
    
    count = defaultdict(int)
    for num in A:
        count[num] += 1
    
    m = 0
    while True:
        if count[m] >= m + 1:
            m += 1
        else:
            break
    print(m)

if __name__ == "__main__":
    main()