#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<int, int>;
using Graph = vector<vector<int>>;
using M = map<int, int>;
using PQ = priority_queue<int>;
using PQG = priority_queue<int, vector<int>, greater<int>>;
const int INF = 1e9;
const int MOD = 1e9 + 7;
const ll LINF = 1e18;
int main() {
  int r;
  cin >> r;
  double ans = 2 * r * M_PI;
  cout << fixed << setprecision(10) << ans << endl;
  return 0;
}
