#include<bits/stdc++.h>
using namespace std;
#define  ft  ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define  ll  long long
#define  pi  pair<int,int>
#define  pb  push_back
#define  F   first
#define  S   second
#define  P   2*acos(0.0)
const int N=2e5+3;
//string s,p;
//vector<int>v;
//map<int,int>mp;
//set<ll>st;
int main()
{
    ft
    double t,i,j,n,m,l,r;
    cin>>l;
    cout<<fixed<<setprecision(20)<<(2.0*P*l)<<"\n";
    return 0;
}

