mod = 998244353


def main():
    import sys
    from collections import Counter
    input = sys.stdin.readline

    N = int(input())
    A = list(map(int, input().split()))

    C = Counter(A)
    flg = 1
    ans = 0
    for a in range(10 ** 5 + 1):
        if C[a] == 0:
            break
        elif C[a] == 1:
            if flg and (a + 1) * 2 <= N:
                flg = 0
                ans += 1
            else:
                break
        else:
            ans += 1
    print(ans)


if __name__ == '__main__':
    main()