N = int(input())
A = list(map(int, input().split()))
M = 101010
D = [0] * M
for a in A:
    D[a] += 1

ans = 0
for i in range(M):
    if D[i] >= 2:
        ans = i + 1
        N -= min(2, D[i])        
    elif D[i] >= 1 and N - D[i] >= 1:
        ans = i + 1
        N -= min(2, D[i])
        D[i + 1] -= 1     
    else:
        break
print(ans)