//   PIGMEISTER™   //

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

#define ll long long
#define ull unsigned long long
#define sidha(i,x,n) for(ll i=(ll)(x);i<(ll)(n);++i)
#define ulta(i,x,n) for(ll i=(ll)(x);i>=(ll)(n);--i)
#define sab(a) a.begin(),a.end()
#define bas(a) a.rbegin(),a.rend()
#define modinv(x) powah(x,mod-2)
#define ncr(n,r) (((fact[n]*modinv(fact[n-r]))%mod*modinv(fact[r]))%mod)
#define factorial fact.pb(1);sidha(i,1,MAX)fact.pb((fact[i-1]*i)%mod)
#define test ll _;cin>>_;while(_--)
#define jaldi ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL)
#define mod (ll)1000000007
//#define mod (ll)998244353
#define MAX 100005

int main()
{
    jaldi;
    
    ll r;
    cin >> r;
    double pi = 3.1415926535;
    cout << (double)2*pi*r;
    
    return 0;
}
//   PIGMEISTER™   //

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

#define ll long long
#define ull unsigned long long
#define sidha(i,x,n) for(ll i=(ll)(x);i<(ll)(n);++i)
#define ulta(i,x,n) for(ll i=(ll)(x);i>=(ll)(n);--i)
#define sab(a) a.begin(),a.end()
#define bas(a) a.rbegin(),a.rend()
#define modinv(x) powah(x,mod-2)
#define ncr(n,r) (((fact[n]*modinv(fact[n-r]))%mod*modinv(fact[r]))%mod)
#define factorial fact.pb(1);sidha(i,1,MAX)fact.pb((fact[i-1]*i)%mod)
#define test ll _;cin>>_;while(_--)
#define jaldi ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL)
#define mod (ll)1000000007
//#define mod (ll)998244353
#define MAX 100005

int main()
{
    jaldi;
    
    ll r;
    cin >> r;
    double pi = 3.1415926535;
    cout << (double)2*pi*r;
    
    return 0;
}
