#include <bits/stdc++.h>
using namespace std;
const int mod = 1e9 + 7;
void solve() {
  double r;
  cin >> r;
  cout << fixed << setprecision(6) << 2.0 * acos(-1) * r;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  int t = 1;
  for (int i = 1; i <= t; ++i) {
    solve();
  }
}
