#include <bits/stdc++.h>
using namespace std;
const int dx[] = {-1, -1, -1, 0, 0, 0, 1, 1, 1};
const int dy[] = {-1, 0, 1, -1, 0, 1, -1, 0, 1};
void fast() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
}
int a[200001];
int main() {
  fast();
  double r;
  cin >> r;
  const double PI = 3.1415926535897931;
  cout << fixed << setprecision(9) << PI * 2 * r;
  return 0;
}
