#include <vector>
#include <iostream>
#include <utility>
#include <algorithm>
#include <string>
#include <deque>
#include <tuple>
#include <queue>
#include <functional>
#include <cmath>
#include <iomanip>
#include <set>
#include <map>
#include <numeric>
#include <unordered_map>
#include <unordered_set>
#include <complex>
#include <array>
//cin.sync_with_stdio(false);
//streambuf
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef pair<double, double> pdd;
using vi = vector<int>;
using vll = vector<ll>;
using vpii = vector<pii>;
using ti3 = tuple<int, int, int>;
using vti3 = vector<ti3>;
template<class T, int s>using va = vector<array<T, s>>;
template<class T, class T2> using umap = unordered_map<T, T2>;
template<class T> using uset = unordered_set<T>;
template<class T> void cmin(T &a, const T&b) { if (a > b)a = b; }
template<class T> void cmax(T &a, const T&b) { if (a < b)a = b; }
#define ALL(a) a.begin(),a.end()
#define rep(i,a) for(int i=0;i<a;i++)
#define rep1(i,a) for(int i=1;i<=a;i++)
const ll mod = 1000000007;
#ifndef INT_MAX
const int INT_MAX = numeric_limits<signed>().max();
#endif
template<class T>using heap = priority_queue<T, vector<T>, greater<T>>;
template<class T>using pque = priority_queue<T, vector<T>, function<T(T, T)>>;
template <class T>
inline void hash_combine(size_t & seed, const T & v) {
	hash<T> hasher;
	seed ^= hasher(v) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
}
namespace std {

	template<typename S, typename T> struct hash<pair<S, T>> {
		inline size_t operator()(const pair<S, T> & v) const {
			size_t seed = 0;
			hash_combine(seed, v.first);
			hash_combine(seed, v.second);
			return seed;
		}
	};
	// Recursive template code derived from Matthieu M.
	template <class Tuple, size_t Index = std::tuple_size<Tuple>::value - 1>
	struct HashValueImpl
	{
		static void apply(size_t& seed, Tuple const& tuple)
		{
			HashValueImpl<Tuple, Index - 1>::apply(seed, tuple);
			hash_combine(seed, std::get<Index>(tuple));
		}
	};

	template <class Tuple>
	struct HashValueImpl<Tuple, 0>
	{
		static void apply(size_t& seed, Tuple const& tuple)
		{
			hash_combine(seed, std::get<0>(tuple));
		}
	};

	template <typename ... TT>
	struct hash<std::tuple<TT...>>
	{
		size_t
			operator()(std::tuple<TT...> const& tt) const
		{
			size_t seed = 0;
			HashValueImpl<std::tuple<TT...> >::apply(seed, tt);
			return seed;
		}

	};
}
ll pow(ll base, ll i, ll mod) {
	ll a = 1;
	while (i) {
		if (i & 1) {
			a *= base;
			a %= mod;
		}
		base *= base;
		base %= mod;
		i /= 2;
	}
	return a;
}
class unionfind {
	vector<int> par, rank, size_;//速度ではなくメモリ効率を考えるならrankのかわりにsizeを使う
public:
	unionfind(int n) :par(n), rank(n), size_(n, 1) {
		iota(ALL(par), 0);
	}
	int find(int x) {
		if (par[x] == x)return x;
		return par[x] = find(par[x]);
	}
	void unite(int x, int y) {
		x = find(x), y = find(y);
		if (x == y)return;
		if (rank[x] < rank[y])swap(x, y);
		par[y] = x;
		size_[x] += size_[y];
		if (rank[x] == rank[y])rank[x]++;
	}
	bool same(int x, int y) {
		return find(x) == find(y);
	}
	int size(int x) {
		return size_[find(x)];
	}
};
ll gcd(ll a, ll b) {
	while (b) {
		ll c = a%b;
		a = b;
		b = c;
	}
	return a;
}
ll lcm(ll a, ll b) {
	return a / gcd(a, b)*b;
}
int popcnt(unsigned long long a) {
	a = (a & 0x5555555555555555) + (a >> 1 & 0x5555555555555555);
	a = (a & 0x3333333333333333) + (a >> 2 & 0x3333333333333333);
	a = (a & 0x0f0f0f0f0f0f0f0f) + (a >> 4 & 0x0f0f0f0f0f0f0f0f);
	a = (a & 0x00ff00ff00ff00ff) + (a >> 8 & 0x00ff00ff00ff00ff);
	a = (a & 0x0000ffff0000ffff) + (a >> 16 & 0x0000ffff0000ffff);
	return (a & 0xffffffff) + (a >> 32);
}
template<class T, class Func = function<T(T, T)>>
class segtree {
	vector<T> obj;
	int offset;
	Func updater;
	T e;
	int bufsize(int num) {
		int i = 1;
		for (; num >i; i <<= 1);
		offset = i - 1;
		return (i << 1) - 1;
	}
	T query(int a, int b, int k, int l, int r) {
		if (r <= a || b <= l)return e;
		if (a <= l && r <= b)return obj[k];
		else return updater(query(a, b, k * 2 + 1, l, (l + r) / 2), query(a, b, k * 2 + 2, (l + r) / 2, r));
	}
public:
	T query(int a, int b) {//[a,b)
		return query(a, b, 0, 0, offset + 1);
	}
	void updateall(int l = 0, int r = -1) {
		if (r < 0)r = offset + 1;
		l += offset, r += offset;
		do {
			l = l - 1 >> 1, r = r - 1 >> 1;
			for (int i = l; i < r; i++)obj[i] = updater(obj[i * 2 + 1], obj[i * 2 + 2]);
		} while (r);
	}
	void update(int k, T &a) {
		k += offset;
		obj[k] = a;
		while (k) {
			k = k - 1 >> 1;
			obj[k] = updater(obj[k * 2 + 1], obj[k * 2 + 2]);
		}
	}
	segtree(int n, T e, const Func &updater_ = Func()) :obj(bufsize(n), e), e(e), updater(updater_) {}
	segtree(vector<T> &vec, T e, const Func &updater = Func()) :obj(bufsize(vec.size()), e), e(e), updater(updater) {
		copy(vec.begin(), vec.end(), obj.begin() + offset);
		updateall();
	}
	typename vector<T>::reference operator[](int n) {
		return obj[n + offset];
	}
};
template<class T>
class matrix {
	vector<vector<T>> obj;
	pair<int, int> s;
public:
	matrix(pair<int, int> size, T e = 0) :matrix(size.first, size.second, e) {}
	matrix(int n, int m = -1, T e = 0) :obj(n, vector<T>(m == -1 ? n : m, e)), s(n, m == -1 ? n : m) {}
	static matrix e(int n) {
		matrix a = (n);
		for (int i = 0; i < n; i++)a[i][i] = 1;
		return a;
	}
	matrix& operator+=(const matrix &p) {
		if (s != p.s)throw runtime_error("matrix error");
		for (int i = 0; i < s.first; i++)
			for (int j = 0; j < s.second; j++)obj[i][j] += p.obj[i][j];
		return *this;
	}
	matrix operator+(const matrix &p) {
		matrix res(*this);
		return res += p;
	}
	matrix& operator-=(const matrix &p) {
		if (s != p.s)throw runtime_error("matrix error");
		for (int i = 0; i < s.first; i++)
			for (int j = 0; j < s.second; j++)obj[i][j] -= p.obj[i][j];
		return *this;
	}
	matrix operator-(const matrix &p) {
		matrix res(*this);
		return res -= p;
	}
	matrix& operator*=(T p) {
		for (auto &a : obj)
			for (auto &b : a)b *= p;
		return *this;
	}
	matrix operator*(T p) {
		matrix res(*this);
		return res *= p;
	}
	matrix operator*(const matrix &p) {
		if (s.second != p.s.first)throw runtime_error("matrix error");
		matrix ret(s.first, p.s.first);
		for (int i = 0; i < s.first; i++)
			for (int j = 0; j < s.second; j++)
				for (int k = 0; k < p.s.second; k++)ret[i][k] += obj[i][j] * p.obj[j][k];
		return ret;
	}
	matrix &operator*=(const matrix &p) {
		return *this = *this*p;
	}
	pair<int, int> size() const {
		return s;
	}
	matrix &mod(T m) {
		for (auto &a : obj)
			for (auto &b : a)b %= m;
		return *this;
	}
	typename vector<vector<T>>::reference operator[](int t) {
		return obj[t];
	}
};
template<class T> inline
matrix<T> pow(matrix<T> &base, unsigned exp) {
	auto base_(base);
	if (base_.size().first != base_.size().second)throw runtime_error("matrix error");
	matrix<T> res = matrix<T>::e(base_.size().first);
	for (;;) {
		if (exp & 1)res *= base_;
		if (!(exp /= 2))break;
		base_ *= base_;
	}
	return res;
}
template<class T> inline
matrix<T> modpow(matrix<T> &base, unsigned exp, T m) {
	auto base_(base);
	if (base.size().first != base_.size().second)throw runtime_error("matrix error");
	matrix<T> res = matrix<T>::e(base_.size().first);
	for (;;) {
		if (exp & 1)(res *= base_).mod(m);
		if (!(exp /= 2))break;
		(base_ *= base_).mod(m);
	}
	return res;
}
struct edge { int to, cap, rev, cost,mf; };

constexpr int MAX_V = 2000;
vector<edge> G[MAX_V];
bool used[MAX_V];
int level[MAX_V];
int iter[MAX_V];

void add_edge(int from, int to, int cap, int cost = 0) {
	G[from].push_back(edge{ to, cap,(int) G[to].size(),cost,cap });
	G[to].push_back(edge{ from,0,(int)G[from].size() - 1,-cost,cap });
}
void bfs(int s) {
	fill_n(level, MAX_V, -1);
	queue<int> que;
	level[s] = 0;
	que.push(s);
	while (!que.empty()) {
		int v = que.front(); que.pop();
		for (int i = 0; i < G[v].size(); i++) {
			edge &e = G[v][i];
			if (e.cap > 0 && level[e.to] < 0) {
				level[e.to] = level[v] + 1;
				que.push(e.to);
			}
		}
	}
}
int dfs(int v, int t, int f) {
	if (v == t)return f;
	for (int &i = iter[v]; i < G[v].size(); i++) {
		edge &e = G[v][i];
		if (e.cap > 0 && level[v] < level[e.to]) {
			int d = dfs(e.to, t, min(f, e.cap));
			if (d > 0) {
				e.cap -= d;
				G[e.to][e.rev].cap += d;
				return d;
			}
		}
	}
	return 0;
}
int max_flow(int s, int t) {
	int flow = 0;
	for (;;) {
		bfs(s);
		if (level[t] < 0)return flow;
		fill_n(iter, MAX_V, 0);
		int f;
		while ((f = dfs(s, t, numeric_limits<int>::max()))>0) {
			flow += f;
		}
	}
}
typedef pair<int, int> P;
int V=MAX_V;
int h[MAX_V];
int dist[MAX_V];
int prevv[MAX_V], preve[MAX_V];

int min_cost_flow(int s, int t, int f) {
	int res = 0;
	fill_n(h, V, 0);
	while (f > 0) {
		priority_queue<P, vector<P>, greater<P>> que;
		fill_n(dist, V, numeric_limits<int>::max());
		dist[s] = 0;
		que.push({ 0,s });
		while (!que.empty()) {
			P p = que.top(); que.pop();
			int v = p.second;
			if (dist[v] < p.first)continue;
			for (int i = 0; i < G[v].size(); i++) {
				edge &e = G[v][i];
				if (e.cap > 0 && dist[e.to]>dist[v] + e.cost + h[v] - h[e.to]){
					dist[e.to] = dist[v] + e.cost + h[v] - h[e.to];
					prevv[e.to] = v;
					preve[e.to] = i;
					que.push({ dist[e.to],e.to });
				}
			}
		}
		if (dist[t] == numeric_limits<int>::max()) {
			return -1;
		}
		for (int v = 0; v < V; v++)h[v] += dist[v];

		int d = f;
		for (int v = t; v != s; v = prevv[v]) {
			d = min(d, G[prevv[v]][preve[v]].cap);
		}
		f -= d;
		res += d*h[t];
		for (int v = t; v != s; v = prevv[v]) {
			edge &e = G[prevv[v]][preve[v]];
			e.cap -= d;
			G[v][e.rev].cap += d;
		}
	}
	return res;
}
int main() {
	int k, t;
	cin >> k >> t;
	int sum = 0, m = 0;
	rep(i, t) {
		int a;
		cin >> a;
		sum += a;
		m = max(m, a);
	}
	int ans = m - (sum - m) - 1;
	cout << (ans > 0 ? ans : 0) << endl;
}

Measured memory_bytes: 256