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 bisect
from itertools import accumulate

N, M = map(int, input().split())
A = list(map(int, input().split()))

A.sort()
#print("A:", A)

L = -1
R = 10 ** 10

while L + 1 < R:
  m = (L + R) // 2
  
  tmp = 0
  for i in range(N):
    tmp += N - bisect.bisect_left(A, m - A[-i])
    #print(tmp)
  if tmp < M:
    R = m
  else:
    if tmp >= M:
      L = m
#print(L, R)

S = list(accumulate([0] + A[::-1]))
#print(S)

cnt = 0
for i in range(N):
  b = N - bisect.bisect_left(A, L - A[i])
  cnt += b
#print(cnt)

answer = 0
C = []
for i in range(N):
  b = N - bisect.bisect_left(A, L - A[i])
  answer +=  S[b] + b * A[i]
  if b:
    C.append(A[-b] + A[i])
  #print(S[b], b, A[i])
C.sort()
if cnt != M:
  answer -= C[0]
print(answer)