#include<bits/stdc++.h>
#define ll long long
#define MP make_pair
#define PB push_back
#define F first
#define S second
#define MOD 1000000000
#define inf -100000
#define fast ios_base::sync_with_stdio(0);cin.tie(NULL);cout.tie(NULL);
using namespace std;
int main()
{
    fast;
   // #ifndef ONLINE_JUDGE
   // freopen("input.txt", "r", stdin);
   // freopen("output.txt", "w", stdout);
    //#endif
    long double r;
    cin>>r;
    cout<<fixed<<setprecision(15)<<2*M_PI*r<<endl;
}	