/*Whose woods these are I think I know.
His house is in the village though;
He will not see me stopping here
To watch his woods fill up with snow.
 
My little horse must think it queer
To stop without a farmhouse near
Between the woods and frozen lake
The darkest evening of the year.
 
He gives his harness bells a shake
To ask if there is some mistake.
The only other sound’s the sweep
Of easy wind and downy flake.
 
The woods are lovely, dark and deep,
But I have promises to keep,
And miles to go before I sleep,
And miles to go before I sleep.*/
 
#include<bits/stdc++.h>
using namespace std;
#define lli long long int
#define pb push_back
#define fi first
#define se second
#define MOD 1000000007
const int maxn= 100010;
int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout<<setprecision(20);
    cout<<fixed;
    int r=0;
    cin>>r;
    cout<<(2.0*M_PI*r)<<endl;
}
