#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using vin = vector<int>;
using vll = vector<long long>;
using vvin = vector<vector<int>>;
using vvll = vector<vector<long long>>;
using vstr = vector<string>;
using vvstr = vector<vector<string>>;
using vch = vector<char>;
using vvch = vector<vector<char>>;
using vbo = vector<bool>;
using vvbo = vector<vector<bool>>;
using vpii = vector<pair<int, int>>;
using pqsin = priority_queue<int, vector<int>, greater<int>>;
int main() {
  int r;
  cin >> r;
  cout << fixed << setprecision((int)10);
  cout << (double)2 * r * 3.1415926535 << endl;
}
