#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef pair<int, int> P;
typedef vector<int> vi;
typedef vector<ll> vll;
// #define int long long
#define pb push_back
#define mp make_pair
#define eps 1e-9
#define INF 2000000000               // 2e9
#define LLINF 2000000000000000000ll  // 2e18 (llmax:9e18)
#define fi first
#define sec second
#define all(x) (x).begin(), (x).end()
#define sq(x) ((x) * (x))
#define dmp(x) cerr << #x << ": " << x << endl;

template <class T>
void chmin(T &a, const T &b) {
  if (a > b) a = b;
}
template <class T>
void chmax(T &a, const T &b) {
  if (a < b) a = b;
}

template <class T>
using MaxHeap = priority_queue<T>;
template <class T>
using MinHeap = priority_queue<T, vector<T>, greater<T>>;
template <class T>
vector<T> vect(int len, T elem) {
  return vector<T>(len, elem);
}

template <class T, class U>
ostream &operator<<(ostream &os, const pair<T, U> &p) {
  os << p.fi << ',' << p.sec;
  return os;
}
template <class T, class U>
istream &operator>>(istream &is, pair<T, U> &p) {
  is >> p.fi >> p.sec;
  return is;
}
template <class T>
ostream &operator<<(ostream &os, const vector<T> &vec) {
  for (int i = 0; i < vec.size(); i++) {
    os << vec[i];
    if (i + 1 < vec.size()) os << ' ';
  }
  return os;
}
template <class T>
istream &operator>>(istream &is, vector<T> &vec) {
  for (int i = 0; i < vec.size(); i++) is >> vec[i];
  return is;
}
void fastio() {
  cin.tie(0);
  ios::sync_with_stdio(0);
  cout << fixed << setprecision(20);
}

namespace Math {
template <int MOD>  // if inv is needed, this shold be prime.
struct ModInt {
  ll val;
  ModInt() : val(0ll) {}
  ModInt(const ll &v) : val(((v % MOD) + MOD) % MOD) {}
  bool operator==(const ModInt &x) const { return val == x.val; }
  bool operator!=(const ModInt &x) const { return !(*this == x); }
  bool operator<(const ModInt &x) const { return val < x.val; }
  bool operator>(const ModInt &x) const { return val > x.val; }
  bool operator>=(const ModInt &x) const { return !(*this < x); }
  bool operator<=(const ModInt &x) const { return !(*this > x); }
  ModInt operator-() const { return ModInt(MOD - val); }
  ModInt inv() const { return this->pow(MOD - 2); }
  ModInt &operator+=(const ModInt &x) {
    if ((val += x.val) >= MOD) val -= MOD;
    return *this;
  }
  ModInt &operator-=(const ModInt &x) {
    if ((val += MOD - x.val) >= MOD) val -= MOD;
    return *this;
  }
  ModInt &operator*=(const ModInt &x) {
    (val *= x.val) %= MOD;
    return *this;
  }
  ModInt &operator/=(const ModInt &x) { return *this *= x.inv(); };
  ModInt operator+(const ModInt &x) const { return ModInt(*this) += x; }
  ModInt operator-(const ModInt &x) const { return ModInt(*this) -= x; }
  ModInt operator*(const ModInt &x) const { return ModInt(*this) *= x; }
  ModInt operator/(const ModInt &x) const { return ModInt(*this) /= x; }
  friend istream &operator>>(istream &i, ModInt &x) {
    ll v;
    i >> v;
    x = v;
    return i;
  }
  friend ostream &operator<<(ostream &o, const ModInt &x) {
    o << x.val;
    return o;
  }
  ModInt pow(ll x) const {
    auto res = ModInt(1ll);
    auto b = *this;
    while (x) {
      if (x & 1) res *= b;
      x >>= 1;
      b *= b;
    }
    return res;
  }
};

template <int MOD>
ModInt<MOD> pow(ModInt<MOD> a, ll x) {
  ModInt<MOD> res = ModInt<MOD>(1ll);
  while (x) {
    if (x & 1) res *= a;
    x >>= 1;
    a *= a;
  }
  return res;
}

constexpr int MOD = 1e9 + 7;
// constexpr int MOD = 998244353;
using mint = ModInt<MOD>;

vector<mint> inv, fac, facinv;
// notice: 0C0 = 1
ModInt<MOD> nCr(int n, int r) {
  assert(!(n < r));
  assert(!(n < 0 || r < 0));
  return fac[n] * facinv[r] * facinv[n - r];
}

void init(int SIZE) {
  fac.resize(SIZE + 1);
  inv.resize(SIZE + 1);
  facinv.resize(SIZE + 1);
  fac[0] = inv[1] = facinv[0] = mint(1ll);
  for (int i = 1; i <= SIZE; i++) fac[i] = fac[i - 1] * mint(i);
  for (int i = 2; i <= SIZE; i++)
    inv[i] = mint(0ll) - mint(MOD / i) * inv[MOD % i];
  for (int i = 1; i <= SIZE; i++) facinv[i] = facinv[i - 1] * inv[i];
  return;
}

template <class T>
int digit(T x) {
  int res = 0;
  while (x) {
    x /= T(10);
    res++;
  }
  return res;
}

template <class T>
int digit_sum(T x) {
  int res = 0;
  while (x) {
    res += x % 10;
    x /= 10;
  }
  return res;
}

template <class T>
T mygcd(T x, T y) {
  if (y == T(0))
    return x;
  else
    return mygcd(y, x % y);
}

template <class T>
T mylcm(T x, T y) {
  assert(x / mygcd(x, y) <= numeric_limits<T>::max() / y);
  return x / mygcd(x, y) * y;
}
}  // namespace Math

template <class T>
vector<pair<T, int>> runLength(vector<T> v) {
  vector<pair<T, int>> res;
  for (int i = 0; i < v.size(); i++) {
    if (res.empty() || res.back().first != v[i])
      res.push_back(make_pair(v[i], 1));
    else
      res.back().second++;
  }
  return res;
}

#define endl "\n"

void solve() {
  int k,t;
  cin >> k >> t;
  vector<int> a(t);
  cin >> a;
  sort(all(a));
  int ans = a.back()-1;
  for(int i=0;i<t-1;i++)ans-=a[i];
  cout << max(0,ans) << endl;
  return;
}

signed main() {
  fastio();
  solve();
  // int t; cin >> t; while(t--)solve();
  return 0;
}


Measured memory_bytes: 256