#include <algorithm>
#include <bits/stdc++.h>
#include <unordered_map>
#include <unordered_set>
using namespace std;
#define mod (int)1000000007
// Big two primes 
#define X 1011001110001111ll
#define M 26388279066623ll
#define all(a) a.begin(),a.end()
#define for0(i, n) for (int i = 0; i < n; i++)
#define for1(i, n) for (int i = 1; i <= n; i++)
#define loop(i,a,b) for (int i = a; i < b; i++)
#define bloop(i,a,b) for (int i = a ; i>=b;i--)
#define tc(t) int t; cin >> t; while (t--)
#define int int
#define ll long long
#define pb emplace_back
#define fio ios_base::sync_with_stdio(false), cin.tie(NULL), cout.tie(NULL)
#define in(x) scanf("%d", &x)
#define rr return 0
#define prec(n) fixed<<setprecision(n)
#define maxpq priority_queue<int>
#define minpq priority_queue<int, vector<int>, greater<int> >
#define inf (int)(1e18)
#define ini(a, i) memset(a, i, sizeof(a))
#define vi vector<int> 
#define fi first
#define se second
#define endl "\n"
#define pi pair<int, int>
#define vii vector<pi>
const int MAXN = (int)((1e5) + 10);
int gcd(int a,int b) { if (a == 0) return b; return gcd(b%a, a);}
int max(int a,int b){if(a>b) return a; else return b;}
int min(int a,int b){if(a<b) return a; else return b;}  
bool isPrime(int N){ for(int i=2;i*i<=N;++i){if(N%i==0) return false;}return true;}
int cbrt(int x){ int lo=1,hi=min(2000000ll,x);while(hi-lo>1){int mid=(lo+hi)/2;if(mid*mid*mid<x){lo=mid;}else hi=mid;}if(hi*hi*hi<=x) return hi;else return lo;} 
bool compare(const pair<int, int> &a, const pair<int, int> &b)
{
  return (a.second < b.second);
}
int main()
{
    fio;
 
    int r;
    cin>>r;

    double d = 2 * (3.14) * r;

    cout<<d<<endl;
    rr;
}