import java.util.*;
import java.lang.*;
import java.io.*;
/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
{
Scanner in
= new Scanner
(System.
in); int k;
double val = 0;
k = in.nextInt();
for (int n = 1; n <= k; n++){
val += 1.0/((3*n - 2)*(3*n + 1));
}
}
}
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