#include <bits/stdc++.h>

using namespace std;

#define PI          3.1415926535897932384626433832795l

#define pb push_back
#define mp make_pair
#define lb lower_bound // First element which is not less than val
#define ub upper_bound // First element greater than val
#define f first
#define s second

#define sz(x) int((x).size())
#define all(x) (x).begin(), (x).end()

using ll = long long;
using vi = vector<int>;
using vvi = vector<vi>;
using vll = vector<ll>;
using vvll = vector<vll>;
using vb = vector<bool>;
using vd = vector<double>;
using vs = vector<string>;
using vpii = vector<pair<int,int>>;
using vpll = vector<pair<ll,ll>>;

using pii = pair<int,int>;
using pll = pair<ll,ll>;

#define endl '\n'
#define re return

#define rep(i,a,b) for (ll i = (a); i < (b); i++)
#define repz(i, a) for (ll i = 0; i < (a); i++)
#define repd(i,a,b) for (ll i = (a); i >= b; i--)
#define repdz(i,a) for (ll i = a; i >=0; i--)
#define trav(a, x) for (auto& a : x)

#define vecin(n,v) for(ll i = 0; i < n; i++) cin>>v[i];
#define llin(a) ll a; cin>>a;
#define llin2(a,b) ll a,b; cin >>a>>b;
#define llin3(a,b,c) ll a,b,c; cin>>a>>b>>c;
#define full(v) v.begin(),v.end()

void solve()
{
    int r;
    cin >> r;
    double f = 2.0*PI*r;
    cout << f << endl;
}

int main()
{
    //ios_base::sync_with_stdio(false);
    //llin(t);
    //while (t--)
    //{
        solve();
    //}
}
