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

B = [0]*(10**5+1)
for a in A:
    B[a] += 1
MEX = -1
for i in range(10**5+1):
    if B[i] == 0:
        MEX = i
        B = B[:i]
        break
if MEX == 0:
    exit(print(MEX))

if 2 <= min(B):
    print(MEX)
elif B.count(1) == 1:
    if 3 <= max(B) or sum(B) < N:
        print(MEX)
    else:
        print(MEX-1)
else:
    cnt = 0
    for i in range(len(B)):
        cnt += B[i] == 1
        if cnt == 2:
            print(i)
            break