const maxn=10000;
var
  start,n,i,j,k,m:integer;
  a:array[0..maxn] of integer;

procedure over;
begin
  writeln('NO');
  halt;
end;
begin
  fillchar(a,sizeof(A),0);
  read(n,m);
  for i:=1 to m do begin
    read(j);
    inc(a[j]);
  end;
  for i:=1 to n do if a[i]>2 then over;
  if (a[0]>1)or(a[n]>1) then over;
  repeat
     start:=-1;
    if a[0]=1 then start:=0 else
      for i:=1 to n-1 do if a[i]=2 then begin start:=i;break;end;
    for i:=start+1 to n-1 do
      if a[i]=0 then begin
        a[start]:=0;
        break;
      end else
      if a[i]>=2 then over;
    if (start=n-1)and(a[n]>0) then over;
    a[start]:=0;
  until (start=n-1)or(start=-1);
  writeln('YES');
end.