/**

author:	©LoneWolfDON
Git: MahinHossainMunna

**/
/*

*/

#include <stdio.h>
#include <math.h>

int main()
{
    float R;

    scanf("%f", &R);

    float pi = 3.1416;

    printf("%f", 2*pi*R); //circumference = 2*pi*R

    return 0;
}