#include <bits/stdc++.h>
#include <set>
#define rep(i,n) for (int i=0; i < (n); ++i)
#define all(a) a.begin(), a.end()
using ll = long long;
using namespace std;
 
// ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }


const double PI = acos(-1);

int main() {
  ll r; cin >> r;
  double ans = 2*r*PI;
  cout << ans << endl;
  return 0;
}