import core.bitop, std.algorithm, std.ascii, std.bigint, std.conv, std.math,
    std.functional, std.numeric, std.range, std.stdio, std.string, std.random,
    std.typecons, std.container, std.format;
static import std.ascii;

// dfmt off
T lread(T = long)(){return readln.chomp.to!T();}
T[] lreads(T = long)(long n){return generate(()=>readln.chomp.to!T()).take(n).array();}
T[] aryread(T = long)(){return readln.split.to!(T[])();}
void scan(TList...)(ref TList Args){auto line = readln.split();
foreach (i, T; TList){T val = line[i].to!(T);Args[i] = val;}}
alias sread = () => readln.chomp();enum MOD = 10 ^^ 9 + 7;
alias PQueue(T, alias less = "a<b") = BinaryHeap!(Array!T, less);
// dfmt on

void main()
{
    long K, T;
    scan(K, T);
    auto A = aryread();
    long prev = -1;
    long ans;
    foreach (i; 0 .. K)
    {
        long m1 = -1, m2 = -1;
        foreach (t; 0 .. T)
        {
            if (m1 == -1 || A[m1] < A[t])
            {
                m1 = t;
            }
            else
            {
                if (m2 == -1 || A[m2] < A[t])
                    m2 = t;
            }
        }
        // writeln(A, prev, " ", m1, " ", m2);
        if (prev == m1)
        {
            if (m2 == -1 || A[m2] == 0)
            {
                ans++, A[m1]--;
                continue;
            }
            A[m2]--;
            prev = m2;
            continue;
        }
        A[m1]--;
        prev = m1;
    }
    writeln(ans);
}
