#include <bits/stdc++.h>
#define rep(i,n) for(long long i = 0; i < (long long)(n); ++i)

using namespace std;

int main() {
	double n;
	cin >> n;
	n = 2 * n * acos(-1);
	cout << n << endl;
	return 0;
}

#include<iostream>
using namespace std;
int main (void)
{
  int r;
  cin>>r;
  r=r*2*3141592;
  cout<<r/1000000<<"."<<r%1000000;
}