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