#include<bits/stdc++.h>
#define pb push_back
#define all(x) (x).begin(),(x).end()
#define rall(x) (x).rbegin(),(x).rend()
#define sz(x)  (long long)(x).size()
#define sqr(x) (x)*(x)
#define sof sizeof
#define ins insert
#define lowerB lower_bound
#define upperB upper_bound
#define mkp make_pair
#define mod 1000000007
#define mod1 998244353
#define N 100003
#define inf 1e18
#define minf -1e18
#define min3(a,b,c) min(a,min(b,c));
#define max3(a,b,c) max(a,max(b,c));
#define rep(i,a,n) for (int i=a;i<n;i++)
#define per(i,a,n) for (int i=n-1;i>=a;i--)
#define FastIO ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL)
#define PRECISION std::cout.unsetf ( std::ios::floatfield );std::cout.precision(15)
#define PI M_PI
#define fi first
#define se second
#define here cout<<"RSSB"<<endl
#define showzero cout<<std::showpoint
#define gcd(a,b) __gcd(a,b)
#define lcm(a,b) (a*b)/gcd(a,b)
#define br "\n"
using namespace  std ;
typedef double db;
typedef long long ll ;
typedef vector<int>vi ;
//typedef vector<l>vl ;
typedef vector<ll>vll ;
typedef vector<vector<ll> >vvl ;
typedef pair<int,int> pii ;
typedef pair<ll,ll> pll ;
typedef vector<pll>vpll;
typedef vector<string>vstr;
typedef vector<bool>vbool ;
/*vbool isprime(N,true);
vll spf(N,0);
vll prime;
void sieve()
{
    isprime[0] = isprime[1] = false;
    for(ll i = 2 ; i<N ; i++)
    {
        if(isprime[i])
        {
            for(ll j=2*i ; j<N ; j+=i)
            {
                isprime[j] = false;
                if(spf[j]==0)
                {
                    spf[j] = i;
                }
            }
            prime.pb(i);
        }
    }
}*/
ll nck(ll n , ll k ,ll m=1e18+7)
{
    ll i , j,b=1,c=1 ;
    if(n-k<k)
    {
        k = n-k ;
    }
    while(k)
    {
        b*=n ;
        c*=k ;
        ll g = __gcd(b,c) ;
        b/=g ;
        c/=g ;
        n--;
        k-- ;
    }
    return(b) ;
}
long long power(long long k , long long n , long long m=mod)
{
    long long res = 1;
    while(n)
    {
        if(n%2!=0)
        {
            res = (res*k)%m ;
        }
        k = (k*k)%m ;
        n = n/2 ;
    }
    return(res) ;
}
void READ(vll &v, ll n)
{
    ll a ;
    for(ll i=0 ; i<n ; i=i+1)
    {
        cin>>a ;
        v.pb(a) ;
    }
}
void PRINT(vll &v , ll a = 0)
{
    //ll a ;
    for(ll i=a ; i<v.size() ; i=i+1)
    {
        v[i] = max((ll)0,v[i]);
        cout<<v[i]<<" " ;
    }
    cout<<"\n" ;
}
ll n,m,k;
bool ok = true;
int main()
{
    FastIO ;
    PRECISION ;
    ll a,b,c,d,e,i,j,t,x,y,z,x1,y1,z1,f;
    //double p,q,r,u,w ;
    char ch,chr ;
    //string str,st,str2,str3;
    //ll a1=0,a2=0,b1=0,b2=0,c1=0 ;
    //cin>>str>>str1;
    cin>>n;
    showzero;
    cout<<(db)2*PI*n;
}