#include<iostream>
using namespace std;
int main (void)
{
  int r;
  cin>>r;
  r=r*2*3141592;
  cout<<r/1000000<<"."<<r%1000000;
}
#include <bits/stdc++.h>
#define rep(i,N) for (int i = 0; i < (N); ++i)
#define all(v) v.begin(), v.end()
using namespace std;
using ll = long long;
using P = pair<int, int>;
int main(void){
  int r;
  cin >> r;

  cout << 2 * 3.141 * r << endl;
}
