from random import getrandbits, randrange from string import ascii_lowercase, ascii_uppercase import sys from math import ceil, floor, sqrt, pi, factorial, gcd, log, log10, log2, inf, cos, sin from copy import deepcopy, copy from collections import Counter, deque, defaultdict from heapq import heapify, heappop, heappush from itertools import ( accumulate, product, combinations, combinations_with_replacement, permutations, ) from bisect import bisect, bisect_left, bisect_right from functools import lru_cache, reduce from decimal import Decimal, getcontext from typing import List, Tuple, Optional inf = float("inf") def ceil_div(a, b): return (a + b - 1) // b def isqrt(num): res = int(sqrt(num)) while res * res > num: res -= 1 while (res + 1) * (res + 1) <= num: res += 1 return res def int1(s): return int(s) - 1 from types import GeneratorType def bootstrap(f, stack=[]): def wrapped(*args, **kwargs): if stack: return f(*args, **kwargs) else: to = f(*args, **kwargs) while True: if type(to) is GeneratorType: stack.append(to) to = next(to) else: stack.pop() if not stack: break to = stack[-1].send(to) return to return wrapped import sys import os from io import BytesIO, IOBase BUFSIZE = 8192 class FastIO(IOBase): newlines = 0 def __init__(self, file): self._fd = file.fileno() self.buffer = BytesIO() self.writable = "x" in file.mode or "r" not in file.mode self.write = self.buffer.write if self.writable else None def read(self): while True: b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE)) if not b: break ptr = self.buffer.tell() self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr) self.newlines = 0 return self.buffer.read() def readline(self): while self.newlines == 0: b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE)) self.newlines = b.count(b"\n") + (not b) ptr = self.buffer.tell() self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr) self.newlines -= 1 return self.buffer.readline() def flush(self): if self.writable: os.write(self._fd, self.buffer.getvalue()) self.buffer.truncate(0), self.buffer.seek(0) class IOWrapper(IOBase): def __init__(self, file): self.buffer = FastIO(file) self.flush = self.buffer.flush self.writable = self.buffer.writable self.write = lambda s: self.buffer.write(s.encode("ascii")) self.read = lambda: self.buffer.read().decode("ascii") self.readline = lambda: self.buffer.readline().decode("ascii") sys.stdin, sys.stdout = IOWrapper(sys.stdin), IOWrapper(sys.stdout) input = lambda: sys.stdin.readline().rstrip("\r\n") print = lambda *args, end="\n", sep=" ": sys.stdout.write( sep.join(map(str, args)) + end ) def II(): return int(input()) def MII(base=0): return map(lambda s: int(s) - base, input().split()) def LII(base=0): return list(MII(base)) def read_graph(n, m, base=0, directed=False, return_edges=False): g = [[] for _ in range(n)] edges = [] for _ in range(m): a, b = MII(base) if return_edges: edges.append((a, b)) g[a].append(b) if not directed: g[b].append(a) if return_edges: return g, edges return g def read_graph_with_weight(n, m, base=0, directed=False, return_edges=False): g = [[] for _ in range(n)] edges = [] for _ in range(m): a, b, w = MII() a, b = a - base, b - base if return_edges: edges.append((a, b, w)) g[a].append((b, w)) if not directed: g[b].append((a, w)) if return_edges: return g, edges return g def yes(res): print("Yes" if res else "No") def YES(res): print("YES" if res else "NO") def pairwise(a): n = len(a) for i in range(n - 1): yield a[i], a[i + 1] def factorial(n): return reduce(lambda x, y: x * y, range(1, n + 1)) def cmin(dp, i, x): if x < dp[i]: dp[i] = x def cmax(dp, i, x): if x > dp[i]: dp[i] = x def alp_a_to_i(s): return ord(s) - ord("a") def alp_A_to_i(s): return ord(s) - ord("A") def alp_i_to_a(i): return chr(ord("a") + i) def alp_i_to_A(i): return chr(ord("A") + i) d4 = [(1, 0), (0, 1), (-1, 0), (0, -1)] d8 = [(1, 0), (1, 1), (0, 1), (-1, 1), (-1, 0), (-1, -1), (0, -1), (1, -1)] def ranges(n, m): return ((i, j) for i in range(n) for j in range(m)) def valid(i, j, n, m): return 0 <= i < n and 0 <= j < m def ninj(i, j, n, m): return [(i + di, j + dj) for di, dj in d4 if valid(i + di, j + dj, n, m)] def gen(x, *args): if len(args) == 1: return [x] * args[0] if len(args) == 2: return [[x] * args[1] for _ in [0] * args[0]] if len(args) == 3: return [[[x] * args[2] for _ in [0] * args[1]] for _ in [0] * args[0]] if len(args) == 4: return [ [[[x] * args[3] for _ in [0] * args[2]] for _ in [0] * args[1]] for _ in [0] * args[0] ] list2d = lambda a, b, v: [[v] * b for _ in range(a)] list3d = lambda a, b, c, v: [[[v] * c for _ in range(b)] for _ in range(a)] class Debug: def __init__(self, debug=False): self.debug = debug cur_path = os.path.dirname(os.path.abspath(__file__)) self.local = os.path.exists(cur_path + "/.cph") def get_ic(self): if self.debug and self.local: from icecream import ic return ic else: return lambda *args, **kwargs: ... ic = Debug(1).get_ic() n, m = MII() a = LII() a.sort() if n % 2 == 1: print(0) exit() res = 0 s = set(a) n2 = n // 2 for ai in a: if ai > n2: break if ai + n2 in s: res += n - 2 print(res)