PROBLEM STATEMENT

The Sieve of Erathosthenes is an ancient method used to find all prime numbers between 2 and a 
given upper bound maxNum, inclusive. It works like this:


make a list of all numbers between 2 and maxNum, inclusive
for x = 2 to maxNum
   if x is not scratched
      for y = 2 to maxNum/x
         if x*y is not scratched, scratch x*y
      end for
   end if
end for


In this fashion, every composite number in the range will get scratched while every prime number 
will remain unscratched.  Return the last number which gets scratched when the Sieve of 
Eratosthenes is run with the given maxNum.


DEFINITION
Class:SieveOfEratosthenes
Method:lastScratch
Parameters:int
Returns:int
Method signature:int lastScratch(int maxNum)


CONSTRAINTS
-maxNum will be between 4 and 2000000000 (2*109), inclusive.


EXAMPLES

0)
18

Returns: 15

When x=2, the numbers 4, 6, 8, 10, 12, 14, 16, and 18 are scratched. When x=3, only 9 and 15 are 
scratched (other multiples like 6 and 12 were already scratched in a previous step). For 
x=5,7,11,13 and 17, there are no new scratches. Therefore, the answer is 15.

1)
5

Returns: 4

2, 3 and 5 are all primes, so the only scratched number is 4.

2)
100

Returns: 91



3)
400

Returns: 361



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