using namespace std;
using ull = unsigned long long;
using ld = long double;
using lll = boost::multiprecision::cpp_int;
using ll = long long;
#define FOR(i, N) for(ll i = 0; i < N; ++i)
 
#define ALL(v) (v).begin(), (v).end()
#define RALL(v) (v).rbegin(), (v).rend()
int main(){
  ll R;
  cin>>R;
  printf("%.5lf\n", R*2*M_PI);
  return 0;
}