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

template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}

#define IOS ios::sync_with_stdio(false), cin.tie(nullptr)
#define FOR(i, a, b) for (int i = (a); i < (int)(b); i++)
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define all(a) (a).begin(), (a).end()
#define allg(a) (a).begin(), (a).end(), greater<int>()
#define MAX(a) *max_element(all(a))

using ll = long long;
using vi = vector<int>;
using vll = vector<ll>;
using vvi = vector<vi>;
using vc = vector<char>;
using vs = vector<string>;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using Graph = vvi;

const int INF = 1001001001;
const int MOD = 1000000007;
const long long INFL = (1LL << 60);
const double EPS = 1e-9;

const int MAX_N = 110;
const int MAX_V = 100010;

int N;
ll W, weight[MAX_N], value[MAX_N];
ll dp[MAX_N][MAX_V];

int main() {
  IOS;
  constexpr char endl = '\n';

  int R;
  cin >> R;
  cout << 2.0 * M_PI * R << endl;
}