output_description: Print the minimum number of days on which the same type of cake as the
  previous day will be eaten.
  
  * * *
input_description: The input is given from Standard Input in the following format:
  
      
      
      K T
      a_1 a_2 ... a_T
attempt: import sys
  import math
  import copy
  import heapq
  from functools import cmp_to_key
  from bisect import bisect_left, bisect_right
  from collections import defaultdict, deque, Counter
  
  sys.setrecursionlimit(1000000)
  
  # input aliases
  input = sys.stdin.readline
  getS = lambda: input().strip()
  getN = lambda: int(input())
  getList = lambda: list(map(int, input().split()))
  getZList = lambda: [int(x) - 1 for x in input().split()]
  
  INF = float("inf")
  MOD = 10**9 + 7
  divide = lambda x: pow(x, MOD - 2, MOD)
  
  
  def nck(n, k, kaijyo):
      return (npk(n, k, kaijyo) * divide(kaijyo[k])) % MOD
  
  
  def npk(n, k, kaijyo):
      if k == 0 or k == n:
          return n % MOD
      return (kaijyo[n] * divide(kaijyo[n - k])) % MOD
  
  
  def fact_and_inv(SIZE):
      inv = [0] * SIZE  # inv[j] = j^{-1} mod MOD
      fac = [0] * SIZE  # fac[j] = j! mod MOD
      finv = [0] * SIZE  # finv[j] = (j!)^{-1} mod MOD
      inv[1] = 1
      fac[0] = fac[1] = 1
      finv[0] = finv[1] = 1
      for i in range(2, SIZE):
          inv[i] = MOD - (MOD // i) * inv[MOD % i] % MOD
          fac[i] = fac[i - 1] * i % MOD
          finv[i] = finv[i - 1] * inv[i] % MOD
  
      return fac, finv
  
  
  def renritsu(A, Y):
      # example 2x + y = 3, x + 3y = 4
      # A = [[2,1], [1,3]])
      # Y = [[3],[4]] または [3,4]
      A = np.matrix(A)
      Y = np.matrix(Y)
      Y = np.reshape(Y, (-1, 1))
      X = np.linalg.solve(A, Y)
  
      # [1.0, 1.0]
      return X.flatten().tolist()[0]
  
  
  class TwoDimGrid:
      # 2次元座標 -> 1次元
      def __init__(self, h, w, wall="#"):
          self.h = h
          self.w = w
          self.size = (h + 2) * (w + 2)
          self.wall = wall
          self.get_grid()
          # self.init_cost()
  
      def get_grid(self):
          grid = [self.wall * (self.w + 2)]
          for i in range(self.h):
              grid.append(self.wall + getS() + self.wall)
  
          grid.append(self.wall * (self.w + 2))
          self.grid = grid
  
      def init_cost(self):
          self.cost = [INF] * self.size
  
      def pos(self, x, y):
          # 壁も含めて0-indexed 元々の座標だけ考えると1-indexed
          return y * (self.w + 2) + x
  
      def getgrid(self, x, y):
          return self.grid[y][x]
  
      def get(self, x, y):
          return self.cost[self.pos(x, y)]
  
      def set(self, x, y, v):
          self.cost[self.pos(x, y)] = v
          return
  
      def show(self):
          for i in range(self.h + 2):
              print(self.cost[(self.w + 2) * i : (self.w + 2) * (i + 1)])
  
      def showsome(self, tgt):
          for t in tgt:
              print(t)
          return
  
      def showsomejoin(self, tgt):
          for t in tgt:
              print("".join(t))
          return
  
      def search(self):
          grid = self.grid
          move = [(0, 1), (0, -1), (1, 0), (-1, 0)]
          move_eight = [
              (0, 1),
              (0, -1),
              (1, 0),
              (-1, 0),
              (1, 1),
              (1, -1),
              (-1, 1),
              (-1, -1),
          ]
          # for i in range(1, self.h+1):
          #     for j in range(1, self.w+1):
          #         cx, cy = j, i
          #         for dx, dy in move_eight:
          #             nx, ny = dx + cx, dy + cy
  
  
  def solve():
      k, t = getList()
      nums = getList()
      mx = max(nums)
      if mx + mx - 1 <= k:
          print(0)
          return
      else:
          print(mx - 1 - (k - mx))
          return
  
  
  def main():
      n = getN()
      for _ in range(n):
          solve()
  
      return
  
  
  if __name__ == "__main__":
      # main()
      solve()
  
problem_description: Statement
  
  There are K pieces of cakes. Mr. Takahashi would like to eat one cake per day,
  taking K days to eat them all.
  
  There are T types of cake, and the number of the cakes of type i (1 ≤ i ≤ T)
  is a_i.
  
  Eating the same type of cake two days in a row would be no fun, so Mr.
  Takahashi would like to decide the order for eating cakes that minimizes the
  number of days on which he has to eat the same type of cake as the day before.
  
  Compute the minimum number of days on which the same type of cake as the
  previous day will be eaten.
samples: [{"input": "7 3\n    3 2 2", "output": "0\n    \n\nFor example, if Mr. Takahashi eats cakes in the order of 2, 1, 2, 3, 1, 3, 1,\nhe can avoid eating the same type of cake as the previous day.\n\n* * *"}, {"input": "6 3\n    1 4 1", "output": "1\n    \n\nThere are 6 cakes. For example, if Mr. Takahashi eats cakes in the order of 2,\n3, 2, 2, 1, 2, he has to eat the same type of cake (i.e., type 2) as the\nprevious day only on the fourth day. Since this is the minimum number, the\nanswer is 1.\n\n* * *"}, {"input": "100 1\n    100", "output": "99\n    \n\nSince Mr. Takahashi has only one type of cake, he has no choice but to eat the\nsame type of cake as the previous day from the second day and after."}]