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

#define MAX(a, b) ((a) > (b) ? (a) : (b))
#define MIN(a, b) ((a) < (b) ? (a) : (b))

int main(void){
    double r;
    scanf("%lf",&r);
    printf("%lf",3.1415*2.0*r);
    return 0; 
}
