#include <bits/stdc++.h>
using namespace std;
void File() {}
void fast() {
  ios::sync_with_stdio(NULL);
  cout.tie(NULL);
  cin.tie(NULL);
}
const double pi = 2 * acos(0.0);
const long long oo = 0x3f3f3f3f;
const int MOD = 1e9 + 7;
const int nn = 2e5 + 15;
int dx[8] = {1, -1, 0, 0, 1, -1, 1, -1};
int dy[8] = {0, 0, 1, -1, 1, -1, -1, 1};
int main() {
  fast();
  double b;
  cin >> b;
  cout << fixed << setprecision(20) << 2 * pi * b << "\n";
  return 0;
}
