#include<bits/stdc++.h>
#define fast ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
#define pb push_back
#define mp make_pair
#define fir first 
#define sec second
#define int long long
#define endl "\n"
# define M_PI 3.14159265358979323846
using namespace std;
const int nax=1e5+5;
vector<int>adj[nax];
const int mod=1e9+7;
const int inf=1e18+7;

signed main(){
	int n;
	cin>>n;
	std::cout << std::setprecision(15) << 2*M_PI*n << endl;
	return 0;
}

