#include <bits/stdc++.h>
#define rep(i,a,b) for(int i=int(a);i<int(b);++i)
#define SIZE 200005
#define INF 1000000005LL
#define MOD 1000000007

using namespace std;
typedef long double ll;
typedef pair <int,int> P;

int main(){

  ll R=0;

  cin >> R;

  cout << setprecision(20) << fixed;
  cout << R*6.28318530717958623200 << endl;

  return 0;
}
