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

ans = [-1]*20
for i in range(N):
    for j in range(20):
        if not 1<<j & (i+1):
            if 1<<j & B[i]:
                exit(print(-1))
            continue
        if ans[j] != -1 and ans[j] != (B[i]>>j & 1):
            exit(print(-1))
        ans[j] = B[i]>>j & 1

n = 0
for i in range(20):
    if ans[i] == 1:
        n |= 1<<i

print(n)