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

#define speed ios_base::sync_with_stdio(0);cin.tie(NULL);cout.tie(NULL);
#define test ll t; cin>>t; while(t--)
#define ll long long
#define endl "\n"
#define all(v) v.begin(),v.end()
#define ff first
#define ss second
#define pb push_back
#define PI 3.1415926535897932384626
#define inf 1e18
#define vll vector<ll>
#define forn(i,n) for (int i = 0; i < int(n); i++)
#define fl(i,a,b) for(ll i=a;i<b;i++)
#define flr(i,a,b) for(ll i=a;i>b;i--)
#define db(x) cout << '>' << #x << ':' << x << endl;
#define print(v) for(auto i=v.begin();i!=v.end();i++){cout<<*i<<" ";}cout<<"\n";
#define MOD 1000000007
const ll N = 2e5;

int main()
{
    speed;
    double l;
    cin>>l;
    cout.precision(20); cout<<fixed; 
    cout<<2*PI*l<<endl;
    return 0;
}
