The law between 0, - 3,8, - 15,24? The 100th number after?

The law between 0, - 3,8, - 15,24? The 100th number after?

Odd bit with positive sign, even bit with negative sign
The first 0 = 1 ^ 2-1;
The second - 3 = - (2 ^ 2-1)
The third 8 = 3 Square-1;
And so on;
The 100th digit is - 9999