import sys
from collections import defaultdict

def main():
    n = int(sys.stdin.readline())
    a = list(map(int, sys.stdin.readline().split()))
    freq = defaultdict(int)
    for num in a:
        freq[num] += 1

    # Compute initial mex
    mex = 0
    while mex in freq:
        mex += 1

    # Find the smallest x where count[x] == 1 and n > 1
    candidate = None
    x = 0
    while True:
        if x in freq and freq[x] == 1 and n > 1:
            candidate = x
            break
        if x > mex:  # No need to check beyond the initial mex
            break
        x += 1

    if candidate is not None:
        print(min(mex, candidate))
    else:
        print(mex)

if __name__ == "__main__":
    main()