#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using vec = vector<ll>;
using mat = vector<vec>;
using vec_b = vector<bool>;
using vec_str = vector<string>;
using graph = mat;
using edge = pair<ll, ll>;
using wgraph = vector<vector<edge>>;
int main() {
  ll R;
  cin >> R;
  cout << setprecision(10) << R * 2 * 3.14159265358979 << endl;
  return 0;
}
