Cartons And Balls

Chef and his friend is playing a game. They have $K$ empty cartons and $N$ balls with them where $N$ is a multiple of $K$ and they need to fill it up with balls. They both can fill $K$ balls in a box in one minute but both of them have different style of filling. Chef puts one ball in each box whereas his friend chooses a box at random having the least number of balls in it and puts $K$ balls int