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

A.sort(reverse = True)
ans = 0
import sys
while A:
    if A[-1] != ans:
        print(ans)
        exit()
    if len(A) <= 1:
        print(ans)
        exit()
    if A[-2] != ans:
        print(ans)
        exit()
    while A and A[-1] == ans:
        A.pop()
    ans += 1
    
print(ans)