N = int(input())
cnt = [0] * (10**5+10)
A = list(map(int,input().split()))
for i in range(N):
    cnt[A[i]] += 1

for i in range(10**5+5):
    if cnt[i] < 2:
        print(i)
        exit()
    cnt[i+1] += cnt[i] - 2