#include <bits/stdc++.h>
using namespace std;
#define ll long long int
#define vi vector<int>
#define pii pair<ll,ll>
#define piii pair<pii, int>
#define pb push_back
#define fs first
#define sc second
#define ppb pop_back
//#define LOCAL
//#define endl "\n"
#define d0(x) cout<<x<<" "
#define d1(x) cout<<(x)<<endl
#define d2(x,y) cout<<(x)<<" "<<(y)<<endl
#define d3(x,y,z) cout<<x<<" "<<y<<" "<<z<<endl
#define d4(x,y,z,w) cout<<x<<" "<<y<<" "<<z<<" "<<w<<endl
#define d5(x,y,z,w,s) cout<<x<<" "<<y<<" "<<z<<" "<<w<<" "<<s<<endl

#define fifo ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define f(i,a,b) for(int (i)=(a);(i)<=(b);++(i))
#define fd(i,a,b) for(int i=(a);i>=(b);--i)
const ll mod = 1e9+7;

// x>>1 is x/2  and x<<1 is x*2 



void solve()
{
	double r;
	cin>>r;
	cout<<fixed<<setprecision(5)<<6.28318530717*r<<endl;

}

int main()
{
	fifo
	int total=1;
	//cin>>total;
	for(int testcase=1;testcase<=total;testcase++)
	{
		solve();
	}



}
