I tried different tests and i don't understand why WA.

Var
   A,B : Array [0..1001] Of Integer;
   N,K,I : Longint;
   Fl : Boolean;

Begin
Read(N);Read(K);
If K>N Then Begin Writeln('NO');Halt;End;
For I:=1 To K Do Begin Read(A[I]);
If A[I]>N Then Begin Writeln('NO');Halt;End;
Inc(B[A[I]]);
End;

If (B[0]=1) And (B[1]=2) Then Begin Writeln('NO');Halt;End;
If (B[N-1]=2) And (B[N]=1) Then Begin Writeln('NO');Halt;End;

For I:=0 To N Do Begin
If B[I]=0 Then Fl:=True;
If (B[I]=2) And not Fl Then Begin Writeln('NO');Halt;End;
End;

For I:=1 To N Do
If (B[I-1]=2) And (B[I]=2) Then Begin Writeln('NO');Halt;End;
Writeln('YES');
End.