void main()
{
    long[] tmp = readln.split.to!(long[]);
    long k = tmp[0], t = tmp[1];
    long[] a = readln.split.to!(long[]);

    long day = 2 * a.reduce!max - a.sum - 1;
    writeln(day > 0 ? day : 0);
}

import std.stdio;
import std.string;
import std.array;
import std.conv;
import std.algorithm;
import std.range;
import std.math;
import std.numeric;
import std.container;
import std.typecons;
import std.ascii;
import std.uni;