import sys

def main():
    N = int(sys.stdin.readline())
    A = list(map(int, sys.stdin.readline().split()))
    freq = {}
    for num in A:
        freq[num] = freq.get(num, 0) + 1
    mex = 0
    while mex in freq:
        mex += 1
    for i in range(mex):
        if freq.get(i, 0) == 1:
            print(mex - 1)
            return
    print(mex)

if __name__ == "__main__":
    main()