#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i = 0; i < n; i++)
#define print(n) cout << n << endl
#define YN(c) (c)?(print("YES")):(print("NO"))
#define Yn(c) (c)?(print("Yes")):(print("No"))
#define yn(c) (c)?(print("yes")):(print("no"))
#define in(v) int v; cin >> v
#define vint vector<int>
#define pb push_back
typedef long long int ll;
#define swap(a,b) do{a=a^b;b=a^b;a=a^b;}while(0)
ll gcd(ll a, ll b){
  if(a<b) return gcd(b,a);
  ll r;
  while(r=a%b){
  	a=b;
  	b=r;
  }
  return b;
}

int main(){
  double r;
  print(2*3.1415*r);
  return 0;
}