#include <bits/stdc++.h>
using namespace std;
#define endl "\n"
#define int long long

int32_t main()
{
      ios::sync_with_stdio(false);
      cin.tie(0); 
      double r;
      cin>>r;
      double p = 22.0/(7.0);
      cout.precision(8); cout<<fixed; 
      cout<<2.0*p*r*1.0<<endl;
      return 0;
}