#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define loop(i,a,n) for(int i=a;i<n;i++)
#define ps(x) cout<<x<<" "
#define pn(x) cout<<x<<"\n"
#define sz(a) int((a).size())
#define pb push_back
typedef vector< int > vi;
typedef pair< int,int > pairs;

int main()
{
 ll n;
 cin>>n;
 cout<<2*3.1428*n;   
}