N=int(input())
B=list(map(int,input().split()))
ans = 0
for j in range(30):
    for i in range(N):
        if (B[i]>>j)&1==1:
            if ((i+1)>>j)&1==0:
                print(-1)
                exit()
            ans |= (1<<j)
if ans == 0:
    assert 1==0
    for i in range(40):
        if (1<<i) > N:
            print(1<<i)
            exit()
for i in range(N):
    if (B[i]&ans) != B[i]:
        print(-1)
        exit()
print(ans)