#include<bits/stdc++.h>
using namespace std;
#define ll long long int
#define ld long double
#define vi vector<int>
#define vll vector<ll>
#define vb vector<bool>
#define vpll vector<pair<ll,ll>>
#define pqi priority_queue<int>
#define pql priority_queue<ll>
#define fi first
#define se second
#define pb push_back
#define pi pair<int,int>
#define pll pair<ll,ll>
#define mii map<int,int>
#define mll map<ll,ll>
#define mib map<int,bool>
#define mlb map<ll,bool>
#define f(i,k,n) for(ll i=k;i<n;i++)
#define trav(a, x) for(auto& a : x)
#define all(x) x.begin(), x.end()
#define endl "\n"
#define IOS ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
const ll mod=1e9+7;
int main()
{
    IOS
    ld r;
    cin>>r;
    ld pii=3.141592653589793238;
    ld ans=2*pii*r;
    cout<<ans<<endl;
}