
#include "bits/stdc++.h"

using namespace std;
#define rep(i, n) for (int64_t i = 0; i < n; i++)

const double PI = 3.14159265358979323846;
int main()
{
        double R;
        cin >> R;
        double ans;
        ans = R * 2 * PI;
        cout << ans << endl;
	
        return 0;
}
