// https://atcoder.jp/contests/agc007/tasks/agc007_c
//
#![allow(unused_imports)]
use std::io::*;
use std::io::Write;
use std::fmt::*;
use std::str::*;
use std::cmp::*;
use std::collections::*;

// Input macros.
// Original by tanakh: https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8
#[allow(unused_macros)]
macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
    ($($r:tt)*) => {
        let s = {
            use std::io::Read;
            let mut s = String::new();
            std::io::stdin().read_to_string(&mut s).unwrap();
            s
        };
        let mut iter = s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
}

#[allow(unused_macros)]
macro_rules! input_inner {
    ($iter:expr) => {};
    ($iter:expr, ) => {};

    ($iter:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($iter, $t);
        input_inner!{$iter $($r)*}
    };

    ($iter:expr, mut $var:ident : $t:tt $($r:tt)*) => {
        let mut $var = read_value!($iter, $t);
        input_inner!{$iter $($r)*}
    };
}

#[allow(unused_macros)]
macro_rules! read_value {
    ($iter:expr, ( $($t:tt),* )) => {
        ( $(read_value!($iter, $t)),* )
    };

    ($iter:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>()
    };

    ($iter:expr, [ next / $t:tt ]) => {
        {
            let len = read_value!($iter, usize);
            (0..len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>()
        }
    };

    ($iter:expr, switch) => {
        {
            let ty = read_value!($iter, i32);
            if ty == 1 {
                vec![ty, read_value!($iter, i32), read_value!($iter, i32)]
            } else if ty == 2 {
                vec![ty, read_value!($iter, i32)]
            } else {
                vec![ty, read_value!($iter, i32)]
            }
        }
    };

    ($iter:expr, chars) => {
        read_value!($iter, String).chars().collect::<Vec<char>>()
    };

    ($iter:expr, usize1) => {
        read_value!($iter, usize) - 1
    };

    ($iter:expr, $t:ty) => {
        $iter.next().unwrap().parse::<$t>().expect("Parse error")
    };
}

#[allow(unused_macros)]
macro_rules! read_line {
    ($t:tt) => {
        {
            let mut s = String::new();
            std::io::stdin().read_line(&mut s).unwrap();
            s.trim_right().parse::<$t>().unwrap()
        }
    }
}

#[allow(unused_macros)]
macro_rules! dvec {
    ($t:expr ; $len:expr) => {
        vec![$t; $len]
    };

    ($t:expr ; $len:expr, $($rest:expr),*) => {
        vec![dvec!($t; $($rest),*); $len]
    };
}

#[allow(unused_macros)]
macro_rules! ifv {
    ($t:expr, $a:expr, $b: expr) => {
        if $t { $a } else { $b }
    }
}

#[allow(unused_macros)]
macro_rules! fill {
    ($t:expr, $v:expr) => {
        for i in 0..$t.len() {
            $t[i] = $v;
        }
    };
}

#[allow(unused_macros)]
macro_rules! join {
    ($t:expr, $glue:expr) => {
        $t.into_iter().map(|w| w.to_string()).collect::<Vec<_>>().join($glue)
    };
}

#[allow(unused_macros)]
macro_rules! debug {
    ($($a:expr),*) => {
        println!(concat!($(stringify!($a), " = {:?}, "),*), $($a),*);
    }
}

// ===

fn doit(a: Vec<(i64, i64)>) -> ((i64, i64), Vec<(i64, i64)>) {
    assert!(a.len() % 2 == 0);

    let n = a.len() / 2;
    let mut sum = (0, 0);
    let mut result = vec![(0i64, 0i64); (n-1)*2];
    for i in 0..n {
        sum.0 += a[i*2].0;
        sum.0 += a[i*2+1].0;
        sum.1 += a[i*2].1;
        sum.1 += a[i*2+1].1;

        let all = (a[i*2].0 + a[i*2+1].0, a[i*2].1 + a[i*2+1].1);

        for l in 0..2 {
            let mut suba = a.clone();
            suba.remove(i*2+1);
            suba.remove(i*2);
            if l == 0 && i >= 1 {
                suba[(i-1)*2+1].0 += all.0;
                suba[(i-1)*2+1].1 += all.1;
            }
            if l == 1 && i+1 < n {
                suba[i*2].0 += all.0;
                suba[i*2].1 += all.1;
            }
            for i in 0..2*n-2 {
                result[i].0 += suba[i].0;
                result[i].1 += suba[i].1;
            }
        }
    }
    (sum, result)
}

fn check(n: usize) {
    let mut initial = vec![];
    for i in 0..2*n {
        initial.push((1, i as i64));
    }
    debug!(doit(initial))
}

fn solve(n: i64, a: f64, d: f64) -> f64 {
    if n <= 1 {
        return (2.0*a+d)/2.0;
    }
    let n = n as f64;
    let a0 = a;
    let an = a+d*(2.0*n-1.0);
    // let add = (a0 + an) * (2.0*n) / 2 / (2*n);
    let add = (a0 + an) / 2.0;

    let div = 2.0*n;
    add + solve((n-1.0) as i64, (2.0*(n+1.0)*a+5.0*d)/div, (n+2.0)*2.0*d/div)
}

fn main() {
    input! {
        n: usize, a: i64, d: i64
    };
    println!("{:.12}", solve(n as i64, a as f64, d as f64));
    // check(5);
}


Measured memory_bytes: 7420