I have seen yellow people who perhaps was grey then they are yellow now , but i wonder is there any red who was been at grey either in his master account or with another account ?

i was trying to solve this problem , http://mirror.codeforces.com/problemset/problem/515/B and i was confused in the part of determining how many days i need to make sure that i make complete test for all friends i think it is confusing part because i see many different formulas n*m*2 , m*m , ( n+m )*n*m ! can any one simply explain it to me ?

as title says , i'm talking about http://mirror.codeforces.com/problemset/problem/327/B problem

that judged as TLE. my submission was only trying to print prime numbers

what are other sequences that aren't prime and aren't divisible terms ?