#include <bits/stdc++.h>
using namespace std;
template <class T>
using V = vector<T>;
template <class T>
using VV = V<V<T>>;
template <class T>
using VVV = V<VV<T>>;
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};
double pi = 3.141592653589793;
long long mod = 1000000007;
int intmax = 2147483647;
int intmin = -2147483648;
long long llmax = 9223372036854775807;
long long llmin = -9223372036854775807;
int iinf = intmax / 8;
long long inf = llmax / 8;
double eps = 1e-12;
void yesno(bool ok, string yes = "Yes", string no = "No") {
  if (ok) {
    cout << yes << endl;
  } else {
    cout << no << endl;
  }
}
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  double R;
  cin >> R;
  cout << 2 * pi * R << endl;
}
