from collections import Counter
n = int(input())
a = list(map(int, input().split()))
m = max(a)
b = [0]*100001
for v in a: b[v] += 1
f = g = False
for i, v in enumerate(b):
    if v == 0:
        if f and not g and i > m: print(i-1); break
        else: print(i); break
    if v == 1:
        if f: print(i); break
        else: f = True
    if v > 2: g = True