#include<bits/stdc++.h>
int main(){
    int n;double d,x,ans=0;std::cin>>n>>d>>x;
    for(;n;--n){
        ans+=d+(n-0.5)*x;
        d=((2*n+2)*d+5*x)/(2*n);
        x=(2*n+4)*x/(2*n);
    }
    printf("%.9f\n",ans);
}