#include<bits/stdc++.h>
#define pb push_back
#define ll long long
#define f(i,a,b) for(int i=a;i<b;i++)
#define f1(i,a,b) for(int i=a;i<=b;i++)
#define w(x) int x; cin>>x; while(x--)
#define mo 998244353
#define d arr0.size()
#define ba back()
using namespace std;
ll p=748683265;
int main(){
	ll r;
  	cin>>r;
  	double l=2*(3.41592653589793238462);
  	double z=l*r;
  	cout<<fixed<<setprecision(21)<<z<<endl;
  return 0;
}
