#include <bits/stdc++.h>

using namespace std;
using ll = long long;
const int mod = 1e9 + 7;

struct mint {
    ll x; // typedef long long ll;
    mint(ll x = 0) : x((x % mod + mod) % mod) {}

    mint operator-() const { return mint(-x); }

    mint &operator+=(const mint a) {
        if ((x += a.x) >= mod) x -= mod;
        return *this;
    }

    mint &operator-=(const mint a) {
        if ((x += mod - a.x) >= mod) x -= mod;
        return *this;
    }

    mint &operator*=(const mint a) {
        (x *= a.x) %= mod;
        return *this;
    }

    mint operator+(const mint a) const { return mint(*this) += a; }

    mint operator-(const mint a) const { return mint(*this) -= a; }

    mint operator*(const mint a) const { return mint(*this) *= a; }

    mint pow(ll t) const {
        if (!t) return 1;
        mint a = pow(t >> 1);
        a *= a;
        if (t & 1) a *= *this;
        return a;
    }

    // for prime mod
    mint inv() const { return pow(mod - 2); }

    mint &operator/=(const mint a) { return *this *= a.inv(); }

    mint operator/(const mint a) const { return mint(*this) /= a; }
};

istream &operator>>(istream &is, const mint &a) { return is >> a.x; }

ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }

struct combination {
    vector<mint> fact, ifact;

    combination(int n) : fact(n + 1), ifact(n + 1) {
        assert(n < mod);
        fact[0] = 1;
        for (int i = 1; i <= n; ++i) fact[i] = fact[i - 1] * i;
        ifact[n] = fact[n].inv();
        for (int i = n; i >= 1; --i) ifact[i - 1] = ifact[i] * i;
    }

    mint operator()(int n, int k) {
        if (k < 0 || k > n) return 0;
        return fact[n] * ifact[k] * ifact[n - k];
    }
};

struct UnionFind {
    vector<int> d;

    UnionFind(int n = 0) : d(n, -1) {}

    int find(int x) {
        if (d[x] < 0) return x;
        return d[x] = find(d[x]);
    }

    bool unite(int x, int y) {
        x = find(x);
        y = find(y);
        if (x == y) return false;
        if (d[x] > d[y]) swap(x, y);
        d[x] += d[y];
        d[y] = x;
        return true;
    }

    bool same(int x, int y) { return find(x) == find(y); }

    int size(int x) { return -d[find(x)]; }
};

ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }

ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }

ll dfs(int n, ll x, ll p, ll b, vector<pair<ll, ll>> &A) {
    if (n < 0)return p;
    ll pp = A[n - 1].first, bb = A[n - 1].second;
    cout << n << " " << p << " " << b << " " << pp + bb + 2 << endl;
    ll res;
    if (x < p + b + pp + bb + 2)res = dfs(n - 1, x, p, b, A);
    else res = dfs(n - 1, x, p + pp + 1, b + bb + 1, A);
    return res;
}


int main() {
    int k, t;
    cin >> k >> t;
    vector<int> m(t);
    for (int i = 0; i < t; ++i) {
        int x;
        cin >> x;
        m[i] = x;
    }
    int post = -1, ans = 0;
    for (int i = 0; i < k; ++i) {
        int x = -1, c = -1;
        for (int j = 0; j < t; ++j) {
            if (j == post || m[j] < 1)continue;
            if (c <= m[j])x = j, c = m[j];
        }
        if (x == -1)x = post;
        if (post == x)ans++;
        m[x]--;
        post = x;
    }
    cout << ans << endl;
    return 0;
}


Measured memory_bytes: 3616