import sys
from collections import defaultdict

def main():
    n, *rest = map(int, sys.stdin.read().split())
    a = rest[:n]
    freq = defaultdict(int)
    for num in a:
        freq[num] += 1
    x = 0
    while True:
        if x not in freq or freq[x] <= 1:
            print(x)
            return
        x += 1

if __name__ == "__main__":
    main()