program aab;
var a,b:longint;
    f:text;
begin
  assign(f,'input.txt');
  reset(f);
  readln(f,a,b);
  close(f);
  assign(f,'output.txt');
  rewrite(f);
  writeln(f,a+b);
  close(f);
end.

Don't bother with files, use standard input/output. This program
should work:

program timus1000(input,output);
var
a,b:longint;
begin
readln(a, b);
writeln(a+b);
end.