#include <iostream>
#include <stdio.h>
#include <vector>

typedef long long ll;

#define rep(i,n) for(int i = 0; i < n; i++)
#define debug(x) cerr << #x << ": " << x << "\n";

using namespace std;
 
const ll INF = ll(1e18)+1;
const double PI=3.14159265358979323846;

int main(){
  double R;
  cin >> R;
  printf("%12.5f", 2*PI*R);
}
