Cho phép viết lại :)
Uses crt;
Var a,b:integer;
Begin
clrscr;
Writeln('Nhap so a = '); Readln(a);
Writeln('Nhap so b = '); Readln(b);
Writeln('Tong hai so a va b la = ',a+b);
Readln;
end.

uses crt;
var a,b,i,s:integer;
begin
clrscr;
write('Nhap a: ');readln(a);
write('Nhap b: ');readln(b);
if (a>b) then
begin
writeln('Gia tri nhap khong thoa man.');
write('Moi nhap lai a: ');readln(a);
write('Moi nhap lai b: ');readln(b);
end;
s:=0;
for i:=a to b do
s:=s+i;
write('Tong cac so nguyen tu ',a,' den ',b,' la: ',s);
readln;
end.