#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int MOD = 1e9 + 7;
const int inf = 1e9;
const ll INF = 1e18;
const double pi = 3.14159265358979323846;
#define rep(i,n) for(int i=0;i<n;i++)

int main(){
	double r;cin>>r;
	cout<<fixed<<setprecision(10)<<2.0*pi*r<<endl;
}