#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using vi = vector<int>;
using vll = vector<ll>;
using P = pair<int, int>;
const int INF = 1e9;
const ll INFLL = 1e18;
const int MOD = 1e9 + 7;
const int NIL = -1;
const ld PI = acosl(-1);
int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  cout << fixed << setprecision(10);
  int r;
  cin >> r;
  cout << 2 * PI * r << endl;
  return 0;
}
