This is my program.

var long:array [1..500,1..3] of integer;
    num:array [1..500,1..2] of integer;
    max,i,j,l,n:integer;

begin
  read(n);
  for i:=1 to n do
  begin
    read(num[i,1],num[i,2]);
    if num[i,1]>num[i,2] then
      begin j:=num[i,1]; num[i,1]:=num[i,2]; num[i,2]:=j; end;
  end;
  for i:=1 to n-1 do
   for j:=i+1 to n do
    if (num[i,1]>num[j,1]) or
     ((num[i,1]=num[j,1]) and (num[i,2]>num[j,2])) then
     begin
       l:=num[i,1]; num[i,1]:=num[j,1]; num[j,1]:=l;
       l:=num[i,2]; num[i,2]:=num[j,2]; num[j,2]:=l;
     end;
  for i:=1 to n do long[i,1]:=num[i,2]-num[i,1];
  for i:=1 to n do long[i,3]:=1;
  for i:=n-1 downto 1 do
    for j:=n downto i+1 do
      if (num[i,2]<=num[j,1]) then
        if long[j,1]+num[i,2]-num[i,1]>long[i,1] then
          begin long[i,1]:=long[j,1]+num[i,2]-num[i,1]; long[i,2]:=j;
long[i,3]:=long[j,3]+1; end;
  max:=1;
  for i:=2 to n do
   if long[i,1]>long[max,1] then max:=i;
  writeln(long[max,3]);
  while long[max,2]<>0 do
   begin
     writeln(num[max,1],' ',num[max,2]);
     max:=long[max,2];
   end;
  writeln(num[max,1],' ',num[max,2]);
end.

> A very careless mistake.It's (-999,999).