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


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


n = int(input())
b = readlist()
bit = ["?"] * 60
for i in range(1, n + 1):
    for j in range(60):
        if i & (1 << j):
            if b[i - 1] & (1 << j) == 0:
                if bit[j] == "Y":
                    print(-1)
                    exit()
                else:
                    bit[j] = "N"
            else:
                if bit[j] == "N":
                    print(-1)
                    exit()
                else:
                    bit[j] = "Y"
ans = 0
for i in range(60):
    if bit[i] == "Y":
        ans |= 1 << i
print(ans)