#include<iostream>
using namespace std;
int main (void)
{
  int r;
  cin>>r;
  r=r*2*3141592;
  cout<<r/1000000<<"."<<r%1000000;
}
#include<bits/stdc++.h>

using namespace std;

#define pi acos(-1)
int main(){
    int r;
    cin >> r;
    cout << fixed << setprecision(20) << (2*r*pi) << endl;
    return 0;
}