Number of Coke

Shannu had been to Goa in his summer holidays. Shannu has to walk a distance of $M$ km to reach his place. As it was too hot Shannu decides to drink a coke after every $N$ km from his start point. At the first $N$ km he drinks 1 coke, at the second $N$ km he drinks 2, at the third $N$ km he drinks 3, he keeps on increasing his drink at every $N$ km till he reaches his destination. How many cokes d