This is my program:
program p1134;

const MaxN    = 1000;

var   N,M,X,i : Integer;
      C       : Array[0..MaxN]of Integer;
      Yes     : Boolean;

begin
  FillChar(C,SizeOf(C),0);
  Read(N,M);
  if (M>0) and (M<=N) then Yes:=true else Yes:=false;
  for i:=1 to M do begin
    Read(X);
    if X>N then Yes:=false else C[X]:=C[X]+1;
  end;
  if (C[0]>1) or (C[N]>1) then Yes:=false;
  for i:=1 to N-1 do begin
    if C[i]>2 then
      Yes:=false else
    if (C[i]=2) and ((C[i-1]>0) or (C[i+1]>0)) then
      Yes:=false;
  end;
  if Yes then Writeln('YES') else Writeln('NO');
end.