Aditi, Bishnu, and Chetna initiated a business with the capital invested by Aditi and Bishnu in a ratio of 9:8. Chetna invested Rs. 800 less than Bishnu. After 18 months, Bishnu and Chetna increased their investment by 25% and 100%, and Chetna doubled her investment. This adjustment led to equal profit shares for Bishnu and Chetna at the end of 3 years. Determine the initial amount invested by Bishnu.