#include <bits/stdc++.h>
using namespace std;
const long long LINF = 1e18;
const int INF = 1e9;
template <class T>
bool chmax(T& a, const T& b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
bool chmin(T& a, const T& b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
using Graph = vector<vector<long long>>;
const long long MOD = 1000000007;
int main(void) {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << setprecision(15);
  double r;
  cin >> r;
  cout << 2.0 * r * 3.1415926535 << endl;
  return 0;
}
