/******************************************
* AUTHOR : cf/villazer *
******************************************/
#include <bits/stdc++.h>
using namespace std;
#define N 100005
#define mod 1000000007
#define dd double
#define rep(i, n) for(int i = 0; i < n; i++)
#define rep1(i,a,b) for(int i=a;i<b;i++)
#define pb push_back
#define mp make_pair
#define clr(x) x.clear()
#define sz(x) ((int)(x).size())
#define F first
#define S second
#define debug(x) cout<<(#x)<<" is "<<(x)<<endl
#define lb(arr,n,x) lower_bound(arr,arr+n,x)-arr
#define ub(arr,n,x) upper_bound(arr,arr+n,x)-arr
#define endl "\n"
#define PI 3.14159265358979323846  /* pi */
#define sqr(x) (x)*(x)
#define all(v)  ((v).begin()),((v).end())
#define degreesToRadians(angleDegrees) (angleDegrees * PI / 180.0) // Converts degrees to radians.
#define radiansToDegrees(angleRadians) (angleRadians * 180.0 / PI) // Converts radians to degrees.

typedef long long ll;
typedef long double ld;
//Calculation of (x^y)%mm
ll mod_exp(ll x,ll y,ll mm)
{
    if(y==0)
        return(1);
    else if(y%2==0)
        return(mod_exp((x*x)%mm,y/2,mm));
    else
        return((x*mod_exp((x*x)%mm,(y-1)/2,mm))%mm);
}


int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);



ll r;
cin>>r;
ld s = 2*PI*r;
cout<<fixed<<setprecision(18)<<s<<endl;

return 0;
}