kvpy.

We form blocks containing 4 successive numbers viz, (i+2i²+3i³+4i⁴) + (5i⁵+..) and so on.

We see that the sum of each block is 2-2i. Let there be such n blocks. So we have

| n(2-2i) | = 18√2

=> (2n)² + (2n)² = 648

=> 8n² = 648

=> n = 9

So there are such 9 blocks viz. 36 numbers.

thanks vivek....
never thought of that ....