#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, n) for (int i = 0; i < n; ++i)
#define rep2(i, i0, n) for (int i = i0; i < n; ++i)

int main() {
  double r;
  cin >> r;
  cout << fixed << setprecision(15);
  cout << 2 * 3.14159265 * r << endl;
  return 0; 
}