#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;
const ll INF = 1001001001;
ll gcd(ll a, ll b) {
  if (a % b == 0) {
    return (b);
  } else {
    return (gcd(b, a % b));
  }
}
ll lcm(ll a, ll b) { return a * b / gcd(a, b); }
int main() {
  cout << fixed << setprecision(10);
  int R;
  cin >> R;
  cout << R * 2 * 6.28318530717958623200 / 2 << endl;
}
