It works correctly on my tests but I always get WA ?!
var
n,m:0..1000;
a:array[1..1000,1..3]of 0..1000;
i,k,z:integer;
begin
readln(n,m);
if (m>n)or (m=0)then
begin
writeln('NO');
halt;
end;
for i:=1 to (n+1) do
begin
a[i,1]:=i-1;
a[i,2]:=i;
end;
z:=n;
for i:=1 to m do
begin
read(k);
if (k<0)or (k>z)then
begin
writeln('NO');
halt;
end;
        if (k<>0)and(k<>z) then
        begin
             if (a[k,3]=0)or(a[k+1,3]=0) then
                         begin
                         a[k,3]:=i;
                         if a[k+1,3]=0 then a[k+1,3]:=i
                         end
        else
                        begin
                        n:=a[k,3];

                        if (a[k-1,3]=n)or(a[k+1,3]=n) then a[k,3]:=i
                                                      else

                                                      begin

                                                      writeln('NO');
                                                      halt;

                                                      end;
                        end;
        end
else if k=0 then
begin
n:=a[1,3];
      if n=0 then a[1,3]:=i
      else
      begin
      if a[2,3]=n then a[1,3]:=i
                  else
                  begin
                  writeln('NO');
                  halt;
                  end;
      end;
end
else if k=z then
     begin
     n:=a[z,3];
     if n=0 then a[z,3]:=i
      else
      begin
      if a[z-1,3]=n then a[z,3]:=i
                  else
                  begin
                  writeln('NO');
                  halt;
                  end;
      end;
     end;
end;
writeln('YES');
end.