The Victory Day of Bangladesh is celebrated on December $16$. Bangladesh achieved victory on December $16$, $1971$. 

$16-12-1971$

Written together is $16121971$, which is a prime number. Niloy wrote all the dates from December $16$, $1971$ to December $16$, $2023$ together (as in $16121971$) on a huge board and then he erased all the prime numbers that he got on the board. Then he calculated the sum of the remaining numbers on the board after erasing prime numbers and then calculated the total sum of digits of the sum. In this way he continued summing the digits until he got a single-digit number. So can you tell what that one-digit number can be?