#include <bits/stdc++.h>
using namespace std;
const long long MOD = 1000000007LL;
const long long INF = 1LL << 60;
using vll = vector<long long>;
using vb = vector<bool>;
using vvb = vector<vb>;
using vvll = vector<vll>;
using vstr = vector<string>;
using pair<long long, long long> = pair<long long, long long>;
using vc = vector<char>;
using vvc = vector<vc>;
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;
}
long long dx[4] = {0, 1, 0, -1};
long long dy[4] = {1, 0, -1, 0};
int main() {
  cout << fixed << setprecision(10);
  long long n;
  cin >> n;
  cout << (2.0 * 3.14 * n) << endl;
}
