#include<bits/stdc++.h>
using namespace std;
#define ll long long int
#define vll vector<ll>
#define vllv vector<vector<ll>> 
#define sz size()
#define pb push_back
#define ff first
#define ss second
#define all(v) v.begin(),v.end()
#define allr(v) v.rbegin(),v.rend()
#define fast ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0)
ll fact(ll n){ll s=1; for(ll i=1;i<=n;i++)s*=i; return s;}
ll ceil1(ll n,ll m){if(n%m==0){return n/m;}return n/m +1;}
vll factor(ll n){
    vll ans;
    if(n>0)ans.pb(1);if(n==2)ans.pb(2);
    for(ll i=2;i*i<=n;i++){if(n%i==0){ans.pb(i);if(n/i!=i)ans.pb(n/i);}}if(n>2)ans.pb(n);
    return ans;
}
ll nCr(ll n, ll r) 
{ 
    return fact(n) / (fact(r) * fact(n - r)); 
} 
void dfs(vllv &v,ll k,vll &par,vll &dis,ll p)
{
 
    par[k]=p;
    dis[k]=dis[p]+1;
    for(ll i=0;i<v[k].sz;i++){
        if(v[k][i]!=p){
            dfs(v,v[k][i],par,dis,k);
        }
    }
}
bool sortbysec(const pair<int,int> &a,const pair<int,int> &b){return (a.second<b.second);} 


void solve()
{
    double n;
    cin>>n;
    cout<<2.0*3.14159265358979323846*n;
    return;
}

int main()
{
    ll q;
    q=1;
    // cin>>q;
    while(q--)
    {
        solve();
    }
    return 0;
}