#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = double;
using pi = pair<int, int>;
using pll = pair<ll, ll>;
using pld = pair<ld, ld>;
using ti3 = tuple<int, int, int>;
using vi = vector<int>;
using vll = vector<ll>;
using vld = vector<ld>;
using vpi = vector<pi>;
using vpll = vector<ll>;
using vpld = vector<ld>;
using vti3 = vector<ti3>;
const double EPS = 1e-9;
const int inf = numeric_limits<int>::max() / 2;
const ll mod = 1e9 + 7;
ll pow(ll base, ll i, ll mod) {
  ll a = 1;
  while (i) {
    if (i & 1) {
      a *= base;
      a %= mod;
    }
    base *= base;
    base %= mod;
    i >>= 1;
  }
  return a;
}
ll gcd(ll a, ll b) {
  while (b) {
    ll c = a % b;
    a = b;
    b = c;
  }
  return a;
}
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }
template <typename T>
bool chmin(T& a, const T& b) {
  if (a > b)
    return a = b, true;
  else
    return false;
}
template <typename T>
bool chmax(T& a, const T& b) {
  if (a < b)
    return a = b, true;
  else
    return false;
}
void solve() {
  int N;
  cin >> N;
  cout << fixed << setprecision(20) << 2 * M_PI * N;
}
int main() {
  cin.tie(NULL);
  solve();
  return 0;
}
