var
   y : array [0..1001] of boolean;
   a : array [1..1001] of integer;
   i, m, n : integer;

   procedure quicksort;
      procedure sort(l, r : integer);
        var
          i1, j1, x, w : integer;
        begin
        i1 := l;
        j1 := r;
        x := a[(l+r) div 2];
          repeat
            while  a[i1]<x do
              inc(i1);
            while  x<a[j1] do
              dec(j1);
            if i1 <= j1 then  begin  w := a[i1]; a[i1] := a[j1];  a[j1] := w;
                                   inc(i1);  dec(j1);     end;
          until i1 > j1;
        if l < j1 then sort(l, j1);
        if i1 < r then sort(i1, r);
        end;
     begin
     sort(1, m);
     end;{quicksort}

begin
   fillchar(y, sizeOf(y), true);
   read(n, m);
   if n>= m then begin
   for i := 1 to m do
   read(a[i]);
   quicksort;
   y[0] := false;
   for i := 1 to m do
     if y[a[i]] then y[a[i]] := false
     else
       if y[a[i]+1] then y[a[i]+1] := false
       else begin writeLn('NO');  halt; end;
   writeLn('YES');
   end
   else writeLn('NO');
end.
i've WA#3, thanks.