#include<iostream>
using namespace std;
int main (void)
{
  int r;
  cin>>r;
  r=r*2*3141592;
  cout<<r/1000000<<"."<<r%1000000;
}
#include <bits/stdc++.h>
using namespace std;
 
#define PI 3.14159265358979323846264338327950L

int main() {
	int R;
	double ans;
	cin >> R;
	ans = R * PI * 2;
	cout << ans << endl;
}