/*  TAHMID RAHMAN
    DAMIAN FOREVER
     MATH LOVER
    NEVER GIVE UP
*/
#include<bits/stdc++.h>
using namespace std;
#define pi acos(-1.0)
#define fastio ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL)
#define ll long long
#define pb push_back
#define fi first
#define se second
#define in insert
#define mp make_pair
#define gcd(a,b) __gcd(a,b);
int main()
{
    fastio;
    double ans,r,p=3.1416;
    cin>>r;
    ans=p*r*2;
    cout<<ans<<endl;

}
