Train to Codetown (Easy)

Codetown has a unique concept of seating people in a train. Each train has N coaches and Coaches are indexed as 1,2,3... N as first , second , third .... Nth coach. For each ith coach if gcd of all pairs in the set {i, i+1 , i+2} is 1 then the ith coach is seated with i people. Else if gcd of any of the pair is not 1, then the coach will be kept empty. Your task is to evaluate the no. of people