#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<ll, ll>;
const int mod = 1e9 + 7;
const ll infll = (1LL << 62) - 1;
const int inf = (1 << 30) - 1;
void YesNo(bool j) {
  cout << (j ? "Yes" : "No") << endl;
  return;
}
void yesno(bool j) {
  cout << (j ? "yes" : "no") << endl;
  return;
}
template <class Head>
void pt(Head&& head) {
  cout << head << endl;
  return;
}
int main() {
  int n;
  cin >> n;
  double a = 2 * n * 3.1415926;
  cout << a << endl;
  return 0;
}
