/*Author SHUBH AGRAWAL  */

#include<bits/stdc++.h>
using namespace std;
#define pb push_back
#define pu push  //  adds the value to the last of the queue 
#define lld long long int
#define ins insert /// used in set to insert the values 
#define adv advance  /// used to increment the iterator
#define mp make_pair
#define fi first
#define se second
#define all(c) c.begin(),c.end()
#define PI 3.1415926
#define INF (lld)1e18
#define vl  vector<long long int >
#define vpl vector< pair<lld ,lld> >
#define vvl vector<vector<lld> > 
#define pll pair <lld,lld>
long long int modp(lld a,lld p,lld n)
{
    int d[100];
    int i,j,k,l;
    if(n==0) return 1;
    for(i=0;n>=1;i++)
    {
        d[i]=n%2;
        n/=2;
    }
    l=i;
    long long int e=1;
    if(d[0]==1) e*=(a%p);
    for(i=1;i<l;i++)
    {
        a*=a;
        a=a%p;
        if(d[i]==1) 
        {
            e*=a;
            e=e%p;
        }
    }
    return e%p;
}
lld modInverse(lld n, lld p) 
{ 
    return modp(n, p, p-2); 
}

long long int nCrModPFermat(lld n, lld r,lld p) 
{ 
   // Base case 
   if (r==0) 
      return 1; 
    
    // Fill factorial array so that we 
    // can find all factorial of r, n 
    // and n-r 
    
    lld fac[n+1]; 
    fac[0] = 1; 
    for (lld i=1 ; i<=n; i++) 
    fac[i] = fac[i-1]*i%p; 
     
    return (fac[n]* modInverse(fac[r], p) % p * 
            modInverse(fac[n-r], p) % p) % p; 
}


long long int gcd(lld b,lld a)
{
    if(a<b) return gcd(a,b);
    if(a%b==0) return b;
    return gcd(a%b , b);
}

lld p1=1000000007,p2=998244353;
lld l,r,mid,ans;
lld n,m,i,j,k;
lld x,y,n1,a,b,n2,h,z,c;
queue<lld> q;
lld d,p=1000000000;
vector< vector <lld > > v(100001);

void bfs(lld x , vector<lld>& d)
{
    d[x]=0;
    q.pu(x);
    while(!q.empty())
    {
        x=q.front();
        q.pop();
        l=v[x].size();
        for(i=0;i<l;i++)
        {
            y=v[x][i];
            if(d[y]==INF) 
            {
                d[y] = d[x]+1;
                q.pu(y);
            }       
        }
    }

}
/*
lld DFS(lld a ,lld x)
{
    visited[a] =true;
    sub_size[a]=1;
 //   if(a==x) 
   //     cont[a] =true;
    lld l= v[a].size();
    for(lld i=0;i<l;i++)
    {
        if(visited[v[a][i]] == false)
        {
            sub_size[a]+= DFS(v[a][i],b);
//            if(cont[v[a][i]] ==true ) cont[a]=true;
        }
    }
    return sub_size[a];

}
*/

void solve()
{
     cin>>n;
     float p=3.14159;
     cout<<2*p*n;
     
}



int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    lld t=1;
//    cin>>t;
    for(int i=0;i<t;i++)
    {
    //    cout<<"Case #"<<i+1<<": ";
        solve();
        cout<<"\n";
    }

    return 0;
}
