#include <bits/stdc++.h>
using namespace std;

int n;
double d0, d;

int main() {
	double ans = 0.0;
	cin >> n >> d0 >> d;
	n = n << 1;
	d0 -= d;
	for( ; n >= 2; n -= 2 ) {
		ans += d0 + d * (n + 1) / 2.0;
		double nd0 = d0 + (2 * d0 + 3 * d) / n;
		double nd = d + (4 * d) / n;
		d0 = nd0, d = nd;
	}
	cout << setprecision(20) << fixed << ans << endl;
}


