#include<bits/stdc++.h>
using namespace std;
typedef long long int ll;
int main()
{
   ios_base::sync_with_stdio(false);
   cin.tie(NULL);
    ll t=1;
    //cin>>t;
    while(t--)
    {
        double n;
        cin>>n;
        double ans=(n*2)*3.14159;
        cout<<ans;
    }
}

