def main():
    with open("input.txt", "r") as file:
        n = int(file.readline())
        ks = [int(i) for i in file.readline().split()]

    ks = sorted(ks, reverse=True)

    c = 0
    for k in ks:
        if c == k:
            return -1
        elif c + 1 > k:
            return c
        else:
            c += 1
    return c


if __name__ == '__main__':
    print(main())