comment on the prev prob

here is the comment of Quan on prev prob solution: http://ideone.com/Lmdgqj

isPrime[i]=true means i is a prime number

and so any number (i*j) is not prime where j>=2

c[x

c[x][i] is the number of numbers n have level equal to k, where 1<=n<=x

so that the answer is c[b][k]-c[a-1][k]