#include<iostream.h>
#include<algorithm.h>
#include<vector.h>
#include<math.h>
using namespace std;
int main(){
    int r;
    scanf("%d",&r);
    double A;
    const double PI  =3.141592653589793238463;
    A = (double)PI * 2 * r;
    printf("%.20f",A);
    return 0;
}