\#include <algorithm>

\#include <iostream>

\#include <string>

using namespace std;

\#define PI 3.14159265359

\#define rep(i, n) for (int i = 0; i < (int)(n); i++)

int main() {  

int R;  

cin >> R;  

double ans = 2  *PI*  R;  

cout << ans << endl;  

return 0;

}