#include <bits/stdc++.h>
using namespace std;
using lint = long long int;
using pint = pair<int, int>;
using plint = pair<lint, lint>;
struct fast_ios {
  fast_ios() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(20);
  };
} fast_ios_;
const lint mod = 1e9 + 7;
int main() {
  int r;
  cin >> r;
  cout << 2 * M_PI * r << "\n";
  return 0;
}
