#include <bits/stdc++.h>
using namespace std;
#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 all(x) (x).begin(),(x).end()
#define sz(x) ((int)(x).size())
#define fi first
#define se second
#define pb push_back
#define vi vector<int >
#define vii vector < pair<int ,int > >
#define IO ios::sync_with_stdio(0) ; cin.tie(0) ; cout.tie(0)
#define IOfiles freopen("input.txt","r+",stdin) ; freopen("output.txt","w+",stdout)
#define PI 3.14159265
const long long mod = 1e9 + 7LL;
typedef long long ll;
typedef pair<int, int> ii;
typedef double db;
ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a;}
ll pwr(ll x , ll n) { ll ans = 1; while (n) {ans *= x; n--;} return ans;}

void solve(){
    
}

int main()
{
    IO;
    double r;
    cin>>r;
    cout<<PI*2*r<<endl;
    return 0;

    int t;
    cin>>t;
    while(t){
        solve();
        t--;
    }
}   