#include <bits/stdc++.h>
using namespace std;
using int64 = long long;
int64 max(int64 a, int64 b) {
  if (b > a)
    return b;
  else
    return a;
}
int64 min(int64 a, int64 b) {
  if (b < a)
    return b;
  else
    return a;
}
signed main() {
  double R;
  cin >> R;
  cout << fixed << setprecision(6);
  cout << R * 2.0 * 3.14159265 << endl;
  return 0;
}
