#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>
#define yes "Yes\n"
#define no "No\n"
int r;
//int m,n;
//int i,j,k,l;
//int a,b,c,d,e,f,g;
//int sum,total;
//long long int ;
//float ;
//double ;
//char a[101],b[101],s[101],t[101],u[101];
int main(void){
    scanf("%d",&r);
    printf("%f",2*r*M_PI);
    return 0;
}