#include <bits/stdc++.h>
using namespace std;
const double PI  = acos(-1.0);

#define print(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl; 
int main()
{
    double x;
    cin >> x;
    cout << 2 * PI * x;
    return 0;
}

