// IlyM

#include <bits/stdc++.h>

#define pb push_back
#define sp " "
#define sl "\n"
#define fi first
#define se second
#define mp make_pair
#define sz(a) ((int)a.size())
#define all(a) a.begin(),a.end()
#define rev(a) reverse(a.begin(),a.end())

using namespace std;

typedef long long ll;
typedef vector<int> vi;
typedef pair<int,int> ii;
typedef vector<ii> vii;
typedef pair<int,ii> iii;

const int N=1e6+5,mod=1000000007;
double r;
int main()
{
	cin.tie(0); cout.tie(0);
	ios_base::sync_with_stdio(0);
	cin>>r;
	r*=3.14159265;
	r*=2;
	cout<<r;
	return 0;
}
