#include <bits/stdc++.h>
using namespace std;
#define fo(i,n) for(int i = 0; i < n; i++)
#define pb push_back
#define all(c) c.begin(),c.end()



int main()
{
    double r;
    cin >> r;
    cout << r*2*3.14 << endl;
    return 0;


}

