#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rrep(i, n) for (int i = 1; i <= (n); ++i)
#define drep(i, n) for (int i = (n)-1; i >= 0; --i)
using namespace std;
typedef long long ll;
typedef vector<int> VI;
typedef vector<VI> VVI;

int main() {
  double r;
  cin >> r;
  cout << fixed;
  cout << setprecision(10);
  cout << r *2* M_PI << endl;
  return 0;
}
