#include <bits/stdc++.h>
template <typename T>
bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T>
bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
using namespace std;
using ll = long long int;
const int MOD = 1000000007;
const int INTMAX = 1001001000;
const ll LLMAX = 1010010010010010000;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  ll R;
  cin >> R;
  printf("%.12f\n", R * 2 * 3.14159265358979323);
  return 0;
}
