#include <bits/stdc++.h>
using namespace std;
void c_p_c() {}
void solve() {
  int n;
  cin >> n;
  double r = 2 * acos(-1.0) * n;
  cout << fixed << setprecision(16) << r << endl;
}
int main(int argc, char const *argv[]) {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  int t = 1;
  for (int i = 0; i < t; i++) solve();
  return 0;
}
