#include<bits/stdc++.h>
using namespace std;
//#define  ONLINE_JUDGE 1
int main()
{
	/*
	#ifdef ONLINE_JUDGE
	freopen("C:/Users/wyx/Desktop/Amy/wangyixuan/c++/test data/data.txt", "r",stdin);
	#endif
	*/
	/*---------------Begin code---------------*/
	int n;
	double m;
	cin>>n;
	m=2*n*3.1415926535898;
	cout<<m;
	/*---------------End  code----------------*/
	/*
	#ifdef ONLINE_JUDGE
	fclose(stdin);
	#endif
	system("pause");
	*/
	return 0;
}
