#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<int, int>;
const int INF = 1000000000;
const int MOD = 1000000007;
ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }
int main() {
  long double r;
  cin >> r;
  long double ans = 2 * r * M_PI;
  cout << fixed << setprecision(10) << ans << endl;
  return 0;
}
