Import igst how to claim ? partially or single

Dear Sir

I Am a Small Importer , I have Questions Pls Clarify 

Goods Imported : 1000 Nos
IGST : 20/- for each
Total IGST Paid : 20000/- 

Date of Import : 5-4-2018
Sold April Month : 500 PCs

Remain Stocks i have : 500 Pcs

Import Goods and  IGST Paid for 1000 Pcs @ 20/-  for Month of APRIL  = 20000/- 
APRIL Month We sold 500 Pcs - So can i Utilise the Complete IGST which i paid for Import 20000/- or 10000/- 

But My total Liability of Tax APRIL : 50000/- ( other items are sold )
so Can i apply 20000/- or 10000/- 

Which is correct Pls Guide me 

How Can i Utilise the IGST which i paid for Import 

Pls Guide me