#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using pint = pair<int, int>;
using pll = pair<ll, ll>;
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int R;
  cin >> R;
  cout << 2 * R * 3.14 << endl;
  return 0;
}
