#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < n; ++i)
#define INF 100100100
using namespace std;
using ll = long long;
using P = pair<int,int>;
using Graph = vector<vector<int>>;

const double pi = acos(-1); 

int main(){
  double r;
  double ans = 2*pi*r;
  cout << fixed << setprecision(10);
  cout << ans << endl;
  return 0;
}
