#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using vl = vector<ll>;
using vc = vector<char>;
using vvl = vector<vl>;
using vvc = vector<vc>;
using pll = pair<ll, ll>;
using vpll = vector<pll>;
double pi = 3.1415926535;
int main() {
  ll R;
  cin >> R;
  cout << setprecision(10) << fixed << 2.0 * (double)R * pi << endl;
  return 0;
}
