#include<bits/stdc++.h>
#define pb push_back
#define en "\n"
#define MOD 1000000007
const double PI  =3.141592653589793238463;
using namespace std;
int main() {
	ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);

	int t=1;
	//cin >> t;
	while (t--) {
		float n;
		cin>>n;
		cout<<2*M_PI*n<<endl;;	
	}
	return 0;
}