#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;
}
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::sync_with_stdio(false), cin.tie(nullptr);
  constexpr char endl = '\n';
  int R;
  cin >> R;
  cout << 2.0 * M_PI * R << endl;
}
