#include<bits/stdc++.h>
using namespace std;
void PDF(double r)
{
    double pi = 3.1416;
    double area;
    area = 2 * pi * r;
    cout << area << endl;
}
int main()
{
    double n,i,j,l;
    cin >> n;
    PDF(n);
    return 0;
}
