N=int(input())
B=[-1]+list(map(int,input().split()))
ans=[-1 for _ in range(len(bin(N)[2:]))]
for i in range(1,N+1):
    for j in range(len(bin(i)[2:])):
        if i>>j&1:
            if B[i]>>j&1:
                if ans[j]==-1:ans[j]=1
                elif ans[j]==1:pass
                else:ans[j]=0
            else:
                if ans[j]==-1:ans[j]=0
                elif ans[j]==0:pass
                else:exit(print(-1))
        else:
            if B[i]>>j&1:
                exit(print(-1))
            else:
                pass
for i in range(len(ans)):
    if ans[i]==-1:ans[i]=0
ans=int("".join(map(str,ans[::-1])),2)
print(ans)