#include<iostream>
#include<algorithm>
#include<functional>
#include<cmath>
#include<string>
#include<vector>
#include<stack>
#include<queue>
#include<map>
#include<set>
#include<deque>
using namespace std;
#define ll long long
const int mod = 1000000007;
const ll INF = 1000000000000000000;


int main()
{
	int R;
	cin >> R;
	double p = 3.1415926535;
	cout << 2.0 * R * p << endl;
}
