#include<iostream>
#include<string>
#include<vector>
#include<queue>
#include<algorithm>
#include<map>
#include<set>
#include<cmath>
typedef long long ll;
#define rep(i,n) for(int i=0;i<(n);i++)
#define pb push_back
#define sz(a) (int)a.size()
using namespace std;

ll INF = 1000000007;

int main() {
	double pi = 3.14159265;
	double n;
	cin >> n;
	cout << 2 * n * pi << endl;
}