#include<stdio.h>

int main()
{
    /*istringstream iss;

    string value = "32 40 50 30 60";
    int arr[5];
    iss.str (value); //what does this do???
    for (int i = 0; i < 5; i++)
    {
        int val;
        iss >> val;
        iss>>arr[i];


        cout << i << "-> " << val <<typeid(val).name()<<endl;
    }
    for(int i=0;i<5;i++){
        cout<<arr[i]<<" ";
    }*/
    int r;
    scanf("%d",&r);
    double ans=2*3.1416*r;
    printf("%f\n",ans);

    return 0;
}
