#include <bits/stdc++.h>
using namespace std;
const int N = 100005;
const double PI = 3.14159265359;
int n;
int a[N];
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  int r;
  cin >> r;
  double ans = 2 * PI * r;
  cout << setprecision(9) << fixed << ans << '\n';
  return 0;
}
