#include<bits/stdc++.h>
#define sutu (ll)1e18+10
#define ll long long
using namespace std;
#define  pi  3.141592653
int main(){
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  ll t,n;
  cin>>t;
  cout<<(2*(pi*t))<<endl;
}
