Problem:
Score : 500 points

Problem Statement

Takahashi has come to a party as a special guest.
There are N ordinary guests at the party. The i-th ordinary guest has a power of A_i.

Takahashi has decided to perform M handshakes to increase the happiness of the party (let the current happiness be 0).
A handshake will be performed as follows:

Takahashi chooses one (ordinary) guest x for his left hand and another guest y for his right hand (x and y can be the same).

Then, he shakes the left hand of Guest x and the right hand of Guest y simultaneously to increase the happiness by A_x+A_y.

However, Takahashi should not perform the same handshake more than once. Formally, the following condition must hold:

Assume that, in the k-th handshake, Takahashi shakes the left hand of Guest x_k and the right hand of Guest y_k. Then, there is no pair p, q (1 \leq p < q \leq M) such that (x_p,y_p)=(x_q,y_q).

What is the maximum possible happiness after M handshakes?

Constraints

1 \leq N \leq 10^5

1 \leq M \leq N^2

1 \leq A_i \leq 10^5

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N M
A_1 A_2 ... A_N

Output

Print the maximum possible happiness after M handshakes.

Sample Input 1

5 3
10 14 19 34 33

Sample Output 1

202

Let us say that Takahashi performs the following handshakes:

In the first handshake, Takahashi shakes the left hand of Guest 4 and the right hand of Guest 4.

In the second handshake, Takahashi shakes the left hand of Guest 4 and the right hand of Guest 5.

In the third handshake, Takahashi shakes the left hand of Guest 5 and the right hand of Guest 4.

Then, we will have the happiness of (34+34)+(34+33)+(33+34)=202.

We cannot achieve the happiness of 203 or greater, so the answer is 202.

Sample Input 2

9 14
1 3 5 110 24 21 34 5 3

Sample Output 2

1837

Sample Input 3

9 73
67597 52981 5828 66249 75177 64141 40773 79105 16076

Sample Output 3

8128170

Solution in Rust (1.15.1) (judge verdict: Accepted)

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)*}
    };
}
macro_rules! input_inner {
    ($iter:expr) => {};
    ($iter:expr, ) => {};
    // var... 変数の識別子, $t...型を一つよむ
    ($iter:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($iter, $t);
        //ここで繰り返し
        input_inner!{$iter $($r)*}
    };
}
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, chars) => {
        read_value!($iter, String).chars().collect::<Vec<char>>()
    };
    ($iter:expr, usize1) => {
        read_value!($iter, usize) - 1
    };
    // 配列の最後のNestではここで型が指定されてparseされる
    ($iter:expr, $t:ty) => {
        $iter.next().unwrap().parse::<$t>().expect("Parse error")
    };
}
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)*}
    };
}

macro_rules! input_inner {
    ($iter:expr) => {};
    ($iter:expr, ) => {};
    // var... 変数の識別子, $t...型を一つよむ
    ($iter:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($iter, $t);
        //ここで繰り返し
        input_inner!{$iter $($r)*}
    };
}

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<_>>()
    };

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

    ($iter:expr, usize1) => {
        read_value!($iter, usize) - 1
    };
    // 配列の最後のNestではここで型が指定されてparseされる
    ($iter:expr, $t:ty) => {
        $iter.next().unwrap().parse::<$t>().expect("Parse error")
    };
}
// =========
pub trait ModI:
    Sized
    + PartialEq
    + Copy
    + std::ops::Add<Output = Self>
    + std::ops::Sub<Output = Self>
    + std::ops::Mul<Output = Self>
    + std::ops::Div<Output = Self>
    + std::ops::AddAssign
    + std::ops::SubAssign
    + std::ops::MulAssign
    + std::ops::DivAssign
    + std::default::Default
    + std::fmt::Display
    + std::fmt::Debug
{
    fn m() -> u64;
    fn new(x: u64) -> Self;
    fn pow(self, n: u64) -> Self;
    fn inv(&self) -> Self;
}
macro_rules! define_modint {
    ($n:ident,$m:expr) => {
        #[derive(Clone, Copy, Eq, PartialEq, PartialOrd, Ord)]
        struct $n(u64);

        #[allow(dead_code)]
        impl ModI for $n {
            fn m() -> u64 {
                $m
            }
            fn new(x: u64) -> $n {
                $n(x % $m)
            }

            fn pow(self, mut n: u64) -> $n {
                let mut ret = $n::new(1);
                let mut base = self;
                while n > 0 {
                    if n & 1 == 1 {
                        ret *= base;
                    }
                    base *= base;
                    n >>= 1;
                }
                ret
            }

            fn inv(&self) -> $n {
                self.pow($m - 2)
            }
        }

        impl std::default::Default for $n {
            fn default() -> $n {
                $n::new(0u64)
            }
        }

        impl std::convert::From<u64> for $n {
            fn from(x: u64) -> $n {
                $n::new(x)
            }
        }

        // Binary operator
        impl std::ops::Add for $n {
            type Output = $n;
            fn add(self, rhs: $n) -> Self::Output {
                $n::new(self.0 + rhs.0)
            }
        }

        impl std::ops::Sub for $n {
            type Output = $n;
            fn sub(self, rhs: $n) -> Self::Output {
                if self.0 >= rhs.0 {
                    $n::new(self.0 - rhs.0)
                } else {
                    $n::new($m - rhs.0 + self.0)
                }
            }
        }

        impl std::ops::Mul for $n {
            type Output = $n;
            fn mul(self, rhs: $n) -> Self::Output {
                $n::new(self.0 * rhs.0)
            }
        }

        impl std::ops::Div for $n {
            type Output = $n;
            fn div(self, rhs: $n) -> Self::Output {
                $n::new(self.0 / rhs.0)
            }
        }

        // Assign
        impl std::ops::AddAssign for $n {
            fn add_assign(&mut self, rhs: $n) {
                *self = *self + rhs;
            }
        }

        impl std::ops::SubAssign for $n {
            fn sub_assign(&mut self, rhs: $n) {
                *self = *self - rhs;
            }
        }

        impl std::ops::MulAssign for $n {
            fn mul_assign(&mut self, rhs: $n) {
                *self = *self * rhs;
            }
        }

        impl std::ops::DivAssign for $n {
            fn div_assign(&mut self, rhs: $n) {
                *self = *self / rhs;
            }
        }

        impl std::fmt::Display for $n {
            fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
                write!(f, "{}", self.0)
            }
        }
        impl std::fmt::Debug for $n {
            fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
                write!(f, "{}", self.0)
            }
        }
    };
}
// 10^8 < p < 10^9
// 3 is primitive p-1 root of these
// 167772161 = 5*2^25 + 1, 469762049 = 7*2^26 + 1, 998244353 = 119*2^23 + 1
// 1224736769 = 73 * 2^24 + 1
// define_modint!(ModInt167772161, 167772161);
define_modint!(ModInt998244353, 998244353);
define_modint!(ModInt1224736769, 1224736769);
fn ntt<T: ModI>(a: &mut [T], n: usize, inv: bool) {
    // h = log2(n)
    let h = {
        let mut i = 0;
        while 1 << i != n {
            i += 1;
        }
        i
    };
    let mut j: usize;
    for i in 0..n {
        j = 0;
        for k in 0..h {
            // (i >> k & 1)はiのk桁目のbit
            // (h - 1 - k)は全体をh-bitとしてk桁目の反対の位置
            j |= (i >> k & 1) << (h - 1 - k);
        }
        // はじめの一回だけひっくりかえす
        if i < j {
            a.swap(i, j)
        };
    }
    // バタフライ演算
    let mut b = 1;
    while b < n {
        let zeta: T = T::new(3).pow((T::m() - 1) / (2 * b as u64));
        for j in 0..b {
            // 3 is primitive root of proth prime
            // 3 ^ ((m - 1) / (n * j)) is primitive n root's j power
            let e: T = if inv {
                zeta.pow(j as u64).inv()
            } else {
                zeta.pow(j as u64)
            };
            let mut k = 0;
            while k < n {
                let s: T = a[j + k];
                let t: T = a[j + k + b] * e;
                a[j + k] = s + t;
                a[j + k + b] = s - t;
                k += b * 2;
            }
        }
        b *= 2;
    }

    if inv {
        let ni = T::new(n as u64).inv();
        for i in 0..n {
            a[i] *= ni;
        }
    }
}

fn mod_conv<T: ModI>(mut a: &mut [T], mut b: &mut [T]) -> Vec<T> {
    let n = a.len();
    // calc each mod
    ntt(&mut a, n, false);
    ntt(&mut b, n, false);
    let mut c = Vec::with_capacity(n);
    for i in 0..n {
        c.push(a[i] * b[i]);
    }
    ntt(&mut c, n, true);
    c
}

fn single_convolution<T: ModI>(a: &mut [T], b: &mut [T]) -> Vec<T> {
    let d: usize = a.len() + b.len() - 1;
    let n = d.checked_next_power_of_two().unwrap();
    let mut a = a.to_vec();
    a.resize(n, T::new(0));
    let mut b = b.to_vec();
    b.resize(n, T::new(0));
    let mut res = mod_conv(&mut a, &mut b);
    res.truncate(d);
    res
}
fn mod_pow(mut a: u64, mut n: u64, m: u64) -> u64 {
    let mut ret = 1;
    while n > 0 {
        if n & 1 == 1 {
            ret *= a;
            ret %= m;
        }
        a *= a;
        a %= m;
        n >>= 1;
    }
    ret
}
// mod mの体におけるaの逆元
fn mod_inv(a: u64, m: u64) -> u64 {
    mod_pow(a, m - 2, m)
}
fn garner(mr: &mut Vec<(u64, u64)>, m: u64) -> u64 {
    mr.push((m, 0));
    // coef... mixed radixの係数, constants... 前まで求めた係数
    let mut coef: Vec<u64> = vec![1; mr.len()];
    let mut constants: Vec<u64> = vec![0; mr.len()];
    for i in 0..mr.len() - 1 {
        let v: u64 = (mr[i].1 + mr[i].0 - constants[i]) * mod_inv(coef[i], mr[i].0) % mr[i].0;
        for j in i + 1..mr.len() {
            constants[j] += coef[j] * v;
            constants[j] %= mr[j].0;
            coef[j] *= mr[i].0;
            coef[j] %= mr[j].0;
        }
    }
    constants[mr.len() - 1]
}

fn main() {
    input! {
        n:usize,
        m:u64,
        a: [usize;n]
    }
    type F0 = ModInt1224736769;
    type F1 = ModInt998244353;
    let mut a = a;
    a.sort();
    let mut a0 = vec![F0::new(0); a[n - 1] + 1];
    let mut a1 = vec![F1::new(0); a[n - 1] + 1];
    let mut b0 = vec![F0::new(0); a[n - 1] + 1];
    let mut b1 = vec![F1::new(0); a[n - 1] + 1];
    for e in a {
        a0[e] += F0::new(1);
        b0[e] += F0::new(1);
        a1[e] += F1::new(1);
        b1[e] += F1::new(1);
    }
    let res0 = single_convolution(&mut a0, &mut b0);
    let res1 = single_convolution(&mut a1, &mut b1);
    let mut ans = 0;
    let mut m = m;
    for i in (0..res0.len()).rev() {
        let mut mr = vec![(1224736769u64, res0[i].0), (998244353u64, res1[i].0)];
        let v = garner(&mut mr, std::u64::MAX);
        if m <= v {
            ans += m * i as u64;
            break;
        }
        ans += v * i as u64;
        m -= v;
    }
    println!("{:?}", ans);
}