There is my code:

Program t1134;

Const MaxN=1000;

Var Card      : array[1..MaxN]of boolean;
    Numb      : array[1..MaxN]of integer;
    Use       : array[1..MaxN]of boolean;
    M,N,i,j   : integer;
    ex        : boolean;

begin
Read(N,M);
if (M>N)or(m=0) then begin
 writeln('NO');
 halt(0);
end;
for i:=1 to N do Card[i]:=false;
for i:=1 to M do Use[i]:=false;
for i:=1 to M do begin
 read(Numb[i]);
 if (Numb[i]<0)or(Numb[i]>n) then begin
   writeln('NO');
   halt(0);
 end;
end;
for i:=1 to M do
 if Numb[i]=0 then begin
  Use[i]:=true;
  if Card[1] then begin
    writeln('NO');
    halt(0);
  end;
  Card[1]:=true;
 end else
 if Numb[i]=n then begin
  Use[i]:=true;
  if Card[n] then begin
    writeln('NO');
    halt(0);
  end;
  Card[n]:=true;
 end;
for i:=1 to m-1 do if not(use[i]) then
 for j:=i+1 to m do if not(use[j]) then
  if numb[i]=numb[j] then begin
   if (card[numb[i]])or(card[numb[j]-1]) then begin
    writeln('NO');
    halt(0);
   end;
   card[numb[i]-1]:=true;
   card[numb[i]]:=true;
   use[i]:=true;
   use[j]:=true;
  end;
repeat
ex:=true;
for i:=1 to m do if not(use[i]) then
 if (card[numb[i]])or(card[numb[i]-1]) then begin
  if (card[numb[i]])and(card[numb[i]-1]) then begin
    writeln('NO');
    halt(0);
  end;
  if card[numb[i]] then begin
   card[numb[i]-1]:=true;
   use[i]:=true;
  end else begin
   card[numb[i]-1]:=true;
   use[i]:=true;
  end;
 end;
until ex;
Writeln('YES');
end.