/*
    author: Salam_35
    Created at :

*/

#include <bits/stdc++.h>
using namespace std;

#define ll long long int
#define pb push_back
#define sc(n) scanf("%d",&n)
#define Max 100005
#define scan(n) scanf("%lld",&n)
#define pi acos(-1.0)
#define _fastio  ios_base:: sync_with_stdio(false); cin.tie(0); cout.tie(0);
#define ALL(x) x.begin(),x.end()
#define sci(n) scanf("%d",&n);
#define scl(n) scanf("%lld",&n);
#define load(array,size)  for(ll i=0 ; i<size ; i++) cin>>array[i] ;
#define load1(array,size)  for(ll i=1 ; i<=size ; i++) cin>>array[i] ;
#define INF	1LL<<19
#define pll pair<ll,ll>
#define mst(arr,value) memset(arr,value,sizeof(arr));
#define tst() int t;cin >> t;while(t--)
#define Mod 100000007
#define YES cout<<"Yes"<<endl;
#define NO cout<<"No"<<endl;
void inversemod(ll n, ll mod, ll invmod[],ll MOD){for(int i=2; i<=n; ++i) invmod[i]=MOD-MOD/i*invmod[MOD%i]%MOD;}
//void bigmod( ll a, ll b ,ll mod){ll r = 1;a = a%mod;while(b){if( b%2 ) r = (r*a)%mod;b = b/2;a = (a*a)%mod; }return (ll)r;}

typedef vector<ll>vec;
typedef pair<ll,ll> pii;
typedef map<ll,ll>mpll;
typedef set<ll>seet;

int main()
{
    double n,m,h;
    string s;
    cin>> n;
    cout<<setprecision(15)<<fixed<<2*pi*n<<endl;
}
