#include <bits/stdc++.h>
using namespace std;
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
int gcd(int a, int b) { return b ? gcd(b, a % b) : a; }
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};
const int INF = 1e9;
const int MOD = 1e9 + 7;
int main() {
  int r;
  cout << (double)r * 2 * 3.141592653 << endl;
}
