import sys
from collections import deque, Counter
sys.setrecursionlimit(5 * 10 ** 5)
from pypyjit import set_param
set_param('max_unroll_recursion=-1')
input = lambda: sys.stdin.readline().rstrip()
ii = lambda: int(input())
mi = lambda: map(int, input().split())
li = lambda: list(mi())
inf = 2 ** 63 - 1
mod = 998244353

n = ii()

A = Counter(li())
yoyu = False
for i in range(0, 10 ** 5 + 2):
    if A[i] >= 3:
        yoyu = True
ans = -1
for i in range(0, 10 ** 5 + 2):
        if A[i] == 1 and yoyu:
            yoyu = False
        elif A[i] <= 1:
            ans = i
            break
k = max(A)
A[k] -= 1
yoyu = True
for i in range(0, 10 ** 5 + 2):
        if A[i] == 1 and yoyu:
            yoyu = False
        elif A[i] <= 1:
            ans = max(ans, i)
            break


print(ans)