#include <bits/stdc++.h>
const int INF = 2147483647;
const long long int MOD = 1000000007;
using namespace std;
using ll = long long;
using P = pair<int, int>;
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
int main() {
  int r;
  cin >> r;
  cout << 2 * r * 3.141592 << endl;
  return 0;
}
