#include<bits/stdc++.h>

using namespace std;

#define ll long long 
#define ld long double
#define pii pair<int,int>
#define pll pair<ll,ll>
#define mkp make_pair
#define pb push_back
#define pf push_front
#define popb pop_back
#define popf pop_front
#define ff first
#define ss second
#define endl "\n"
#define MOD 1000000007
#define MOD1 998244353

ll GCD(ll x,ll y){if(y==0)return x;return GCD(y,x%y);}
ll LCM(ll x,ll y){return (x*y)/(GCD(x,y));}
ll LOGK(ll x,ll k){if(x>=k)return 1+LOGK(x/k,k);return 0;}
ll MPOW( ll a, ll b, ll m) { if(b==0) return 1; ll x=MPOW(a,b/2,m); x=(x*x)%m; if(b%2==1) x=(x*a)%m; return x;}
ll MINV(ll a,ll m) {return MPOW(a,m-2,m);}

class pnc{
  ll FACT_MAX,MODU;
  vector<ll> fact;
public:
  pnc( ll n, ll m) { FACT_MAX = n; fact.resize(FACT_MAX); MODU = m; MFACT_INIT(MODU);}
  void MFACT_INIT( ll m) { fact[0]=1; for(ll i=1;i<FACT_MAX;++i) fact[i]=(i*fact[i-1])%MODU;}
  ll MFACT( ll n) { return fact[n];}
  ll PERM( ll n, ll r) { return (fact[n]*::MINV(fact[n-r],MODU))%MODU;}
  ll COMB( ll n , ll r) { return (PERM(n,r)*::MINV(fact[r],MODU))%MODU;}
};

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);    

    ll test=1;
    //cin>>test;
    while(test--)
    {
        ll r;
        cin>>r;

        cout<<setprecision(10);

        ld ans=(2*22*r)/7.0;

        cout<<ans;
    }
}