#include<stdlib.h>
#include<assert.h>
#include<stdio.h>



#define ll long long
#define MAX_N 300001

/*
_
_._ _..._ . - ',     _.._(`))
'-. `     ' / -._. - '    ', /
)         \            '.
/ _    _ | \
| a    a    /              |
\   . - .;
'-('' ).-', '       ;
'-;           |      .'
\           \    /
| 7.__  _. - \   \
| |  |  ``/ / `  /
/, _|  |   /, _/   /
/, _ / '`-'
*/
using namespace std;





int main()
{
	int c;
	cin >> c;
	cout << 2 * 3.14159 * c;

}

		
