#include <bits/stdc++.h>
using namespace std;
#define IOS ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
#define ll long long
#define vll vector<ll>
#define vi vector<int>
#define pb push_back
#define fi first
#define se second
#define pii pair<int,int>
#define pll pair<ll,ll>
#define mii map<int,int>
#define mll map<ll,ll>
#define all(x) (x).begin(),(x).end()
#define S(x) (int)(x).size()
#define L(x) (int)(x).length()
#define trace(x) cerr<<#x<<" is "<<x<<'\n';

const int mod = 1e9+7;
const ll infl = 0x3f3f3f3f3f3f3f3fLL;
const int infi = 0x3f3f3f3f;

void solve()
{
	long double r;
	cin>>r;
	cout<<r*M_PIl*2;
}

int main()
{
	IOS
	int t=1;
	//cin>>t;
	while(t--)
	{
		solve();
	}
}
