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

typedef unsigned long long int ull;
typedef long long int ll;
typedef long double ld;

int main(void) {
  int scan; // scanf result
  double answer; // to output 
  int n; // to input 

  scan = scanf("%d", &n);

  answer = n * 2 * 3.141592;

  printf("%lf", answer);
  return (0);
}
