#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef long double ld;
typedef pair<int,int> Pii;
typedef pair<ll, ll> Pll;
typedef pair<int, Pii > P1;

#define rep(i,n) for(int i=0;i<n;i++)
#define sor(v) sort(v.begin(),v.end())
#define rev(s) reverse(s.begin(),s.end())
#define lb(vec,a) lower_bound(vec.begin(),vec.end(),a)
#define ub(vec,a) upper_bound(vec.begin(),vec.end(),a)
#define mp1(a,b,c) P1(a,P(b,c))
#define get_arsize(a) (sizeof(a)/sizeof(a[0]))
#define PI 3.14159265359
const ll MOD = 1000000007;
const double EPS = 1e-10;
const int MAX = 200000;
// 20 mins delayed
int main() {
  double n;
  cin >> n;

  cout << setprecision(5) << n * (double)2 * PI << endl;
  return 0;
}
