import sys
from itertools import combinations
input = sys.stdin.readline

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

dp = [0]*(1 << N)

for bitset in range(1 << N):
    for (p1, power1), (p2, power2) in combinations(enumerate(power), r=2):
        if ((1 << p1) | (1 << p2)) & bitset:
            continue
        dp[bitset | (1 << p1) | (1 << p2)] = max(
            dp[bitset | (1 << p1) | (1 << p2)],
            dp[bitset] + (power1 ^ power2)
        )

print(dp[-1])