#include <bits/stdc++.h>
using namespace std;
#define fast_io ios_base::sync_with_stdio(false);cin.tie(NULL)
 
void solve()
{
   int r;
   cin >> r;
   double cur=2.0*M_PI*r;
    cout << setprecision(10) << cur << endl;
}
 
 
int main()
{
    fast_io;
    int t;
    //cin >> t;
t=1;
    for (int tt=1;tt<=t;tt++)
    {
        solve();
        
 
    }
 
    
    return 0;
}
 
 
#include <bits/stdc++.h>
using namespace std;
#define fast_io ios_base::sync_with_stdio(false);cin.tie(NULL)

void solve()
{
   int r;
   cin >> r;
   double cur=2.0*M_PI*r;
    cout << setprecision(10) << cur << endl;
}


int main()
{
    fast_io;
    int t;
    //cin >> t;
t=1;
    for (int tt=1;tt<=t;tt++)
    {
        solve();
        

    }

    
    return 0;
}


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