#include <bits/stdc++.h>
#include <math.h>
 
#define rep(i, n) for(int i = 0; (int)(i) < n; i++)
 
using namespace std;
 
int main() {
  int R;
  cin >> R;
  float ans;
  ans = 2 * 3.14 * R;
  cout << ans << endl;
}
  