SGU 105

There is sequence 1, 12, 123, 1234, ..., 12345678910, ... . Given first N elements of that sequence. You must determine amount of numbers in it that are divisible by 3.

Input

Input contains N (1<=N<=2³¹ - 1).

Output

Write answer to the output.

Sample Input

4

Sample Output

2

找规律易知会出现011 011 ..... 011 的情况（0代表不可div,1代表可div) 推公式直接输出即可。

 #include <iostream>
 #include <cstdio>

 using namespace std;

 long long n;
 int a[] = {,,};
 int main()
 {
     scanf("%I64d",&n);

     printf("%d\n",n /  *  + a[n % ] );
     return ;
 }