#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define rep(i,n) for(long long (i)=0;(i)<(n);++(i))
#define ref(i,a,b) for (long long (i)=(a); (i)<=(b); ++(i))
#define endl '\n'
#define st first
#define nd second
#define pb push_back
#define mp make_pair
const int mx=2e6+5;

int main(){
	ios_base::sync_with_stdio(false); cin.tie(0);
	double r;cin >> r;
	cout << M_PI*2*r << endl;

}
	
