def readint():
    return map(int, input().split())


def readlist():
    return list(readint())


n = int(input())
b = readlist()
ans = 0
for i in range(n):
    for j in range(60):
        if b[i] & (1 << j):
            ans |= 1 << j
for i in range(1, n + 1):
    for j in range(60):
        if b[i - 1] != (i & ans):
            print(-1)
            exit()
print(ans)