#include <bits/stdc++.h>
using namespace std;
using p = pair<int, int>;
using P = pair<long long, long long>;
using V = vector<int>;
using VV = vector<vector<int>>;
using VVV = vector<vector<vector<int>>>;
using VL = vector<long long>;
using VVL = vector<vector<long long>>;
using VVVL = vector<vector<vector<long long>>>;
using Graph = VV;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  double r;
  cin >> r;
  cout << r * 3.14159265359 << endl;
}
