#include<algorithm>
#include<cmath>
#include<cstring>
#include<cstdio>
#include<iostream>
#include<vector>
#include<queue>
#include<deque>
#include<stack>
#include<set>
#include<map>
#include<string>
#include <bits/stdc++.h>
#define rep(i, n) for(int i = 0; i < (int)n; i++)
#define ll long long 
using namespace std;
int main(){
  ll n;
  cin >>n;
  cout <<fixed<<setprecision(5)<<n*2*3.141592<<endl;
}