N = int(input())
A = list(map(int, input().split()))
maxa = max(A)
X = [0] * (maxa + 1)
for a in A:
    X[a] += 1
cnt = 0
for i in range(maxa + 1):
    if X[i] == 0:
        i -= 1
        break
    elif X[i] == 1:
        if cnt == 0:
            cnt += 1
        else:
            i -= 1
            break

if cnt == 0:
    print(i + 1)
else:
    if sum(X[:i + 1]) >= 2 * (i + 1):
        print(i + 1)
    elif N >= 2 * (i + 1):
        print(i + 1)
    else:
        print(i)