program prison;
var
  p,q,i,k,d,j:longint;
  hash:array[1..1000]of boolean;
  biaozhi:array[1..1000,1..1000]of boolean;
function sf(x,y:longint):longint;
var
  i:integer;
  min,t:longint;
  y1,y2:longint;
begin
  min:=maxlongint;
  t:=0;
  for i:=x to y do
    if hash[i] then
    begin
      y1:=0;
      y2:=0;
      if biaozhi[x,i-1] then
        y1:=sf(x,i-1);
      if biaozhi[i+1,y] then
        y2:=sf(i+1,y);
      t:=y1+y2+y-x;
      if t<min then
        min:=t
    end;
  sf:=min
end;
begin
  assign (input,'prison.in');
  reset (input);
  assign (output,'prison.out');
  rewrite (output);
    readln (p,q);
    fillchar(hash,sizeof(hash),false);
    fillchar(biaozhi,sizeof(biaozhi),false);
    for i:=1 to q do
    begin
      read (d);
      hash[d]:=true;
      biaozhi[d,d]:=true
    end;
    for i:=1 to p-1 do
      for j:=i+1 to p do
        if hash[j] then
          biaozhi[i,j]:=true
        else
          biaozhi[i,j]:=biaozhi[i,j-1];
    k:=sf(1,p);
    writeln (k);
  close (input);
  close (output)
end.