#include <bits/stdc++.h>
using namespace std;
double solve(int n, double d, double x) {
	if (n == 0) return 0;
	double res = d + (2 * n - 1) * x / 2;
	double dd = ((2 * n + 2) * d + 5 * x) / 2 / n;
	double xx = (n + 2) * x / n;
	return res + solve(n - 1, dd, xx);
}
int main() {
	int n, d, x;
	scanf("%d%d%d", &n, &d, &x);
	double ans = solve(n, d, x);
	printf("%.10f\n", ans);
}

