#include <bits/stdc++.h>
using namespace std;
long long mod_exp(long long x, long long y, long long mm) {
  if (y == 0)
    return (1);
  else if (y % 2 == 0)
    return (mod_exp((x * x) % mm, y / 2, mm));
  else
    return ((x * mod_exp((x * x) % mm, (y - 1) / 2, mm)) % mm);
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long r;
  cin >> r;
  long double s = 2 * 3.14159265358979323846 * r;
  cout << fixed << setprecision(18) << s << "\n";
  return 0;
}
