#include <bits/stdc++.h>

using namespace std;

double f(int n, double d, double x) {
    if(n == 0) return 0.0;
    else return f(n-1, ((2*n+2)*d + 5*x)/(2*n), ((n+2)*x)/n) + d + (x*(2*n-1)/2); 
} 

int N, d, x;

int main() {
    cin >> N >> d >> x;
    cout << setprecision(12) << f(N, (double)d, (double)x) << endl;
}