#include <iostream>
#include <map>
#include <cmath>
#include <queue>
#include <string>
#include <vector>
#include <algorithm>
#include <functional>
using namespace std;
#pragma warning (disable: 4996)

int main() {
	double PI = 3.14159265358979, R; cin >> R;
	printf("%.12lf\n", 2.0 * R * PI);
	return 0;
}
#include<iostream>
using namespace std;
int main (void)
{
  int r;
  cin>>r;
  r=r*2*3141592;
  cout<<r/1000000<<"."<<r%1000000;
}