CF390B Inna, Dima and Song

Inna is a great piano player and Dima is a modest guitar player. Dima has recently written a song and they want to play it together. Of course, Sereja wants to listen to the song very much. A song is a sequence of notes. Dima and Inna want to play each note at the same time. At that, they can play the $ i $ -th note at volume $ v $ ( $ 1

The first line of the input contains integer $ n $ $ (1

In a single line print an integer — the maximum possible joy Sereja feels after he listens to a song.

In the first sample, Dima and Inna play the first two notes at volume $ 1 $ ( $ 1+1=2 $ , the condition holds), they should play the last note at volumes $ 1 $ and $ 2 $ . Sereja's total joy equals: $ 1·1+1·1+1·2=4 $ . In the second sample, there is no such pair $ (x,y) $ , that $ 1