javascript - how change price value according to litre or kg

Question

Hai ,

    any one give sample to me to know how to code add to cart

 

 

 javascript - how to  change price value according to litre or kg.

 

 

i have option menu it has product 1litre, 2 litre  and i have input box to code the price  according to 1lt or 2 lt.

 

now tell me how to change  the price when i choose 1 lt it need to show price in input box and i choose 2 lt it need to show another price in input box .