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

int n;
double d, x, ans, t;

int main () {
	scanf("%d%lf%lf", &n, &d, &x);
	while (n) {
		ans += (2.0 * n - 1) / 2.0 * x + d;
		t = 1.0 / n;
		d += d * t + (t * 0.5 - 1) * x;
		x += x * t * 2;
		d += x;
		--n;
	}
	printf("%.15f\n", (double)ans);
	return 0;
}
/*
2 1 1
*/