Divisibility by 3

Let's consider a sequence $1,12,123,1234,12345,123456,1234567,12345678,123456789,12345678910,1234567891011,...$ .

Write a program that determines how many elements of this sequence among first $n$ are divisible by three.

Input

One positive integer $n(1\le n\le 2^{31}-1)$.

Output

Print one number — the answer to the problem.

Examples