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 PyPy3 (2.4.0) (judge verdict: Wrong Answer)

import itertools
import bisect
n, m = list(map(int, input().split()))
al = list(map(int, input().split()))
al.sort()
ub = al[-1]*2
lb = 0
while lb + 1 < ub:
    target = (ub+lb)//2
    tmp = 0
    for a in al:
        sa = target - a
        # リストAに値xを入れ、xが複数になるとき、一番左の値xのインデックスを返す
        i = bisect.bisect_left(al, sa)
        tmp += (n-i)
    if (tmp >= m):
        lb = target
    else:
        ub = target

target = ub
rui_al = [0] + list(itertools.accumulate(al))
ans = 0
tmp = 0
for a in al:
    sa = target - a
    # リストAに値xを入れ、xが複数になるとき、一番左の値xのインデックスを返す
    i = bisect.bisect_left(al, sa)
    tmp += (n-i)
    ans += (rui_al[-1] - rui_al[i])
    ans += a*(n-i)



ans += (m-tmp)*target
print(ans)