Chefu and Tiles

Chefu is an interior designer and often gets a contract to decorate the floor of the house with tiles. Chefu is in demand because of his unusual design.If $N$ is the total no of tiles required for the floor, for every $i\leq N$ where i and N are co-prime,Chefu puts a Black tile or else White tile. As Chefu is in demand, he want u to help him with number of Black tile needed for the floor. Inp