#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define pi 2*acos(0.0)
#define f first
#define s second
#define pb push_back
#define gap ' '
#define fastio {ios_base::sync_with_stdio(false);cin.tie(NULL);}
#define mod 1000000007

int main()
{
    double r;
    cin>>r;
    double ans=2*pi*r;
    cout<<setprecision(10)<<fixed<<ans<<endl;
}
