#define ll long long
using namespace std;
const int maxn = 2e5+5;
//inline void fastio() {ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);}
 
int _;
 
int main()
{
    //fastio();
    double r;
    scanf("%lf",&r);
    printf("%.8f\n",r*2*acos(-1));
    return 0;
}