1:
uses crt;
var a:integer;
begin
clrscr;
repeat
write('Nhap a='); readln(a);
if a<=0 then writeln('Nhap lai');
until a>0;
readln;
end.
2:
uses crt;
var a:integer;
begin
clrscr;
repeat
write('Nhap a='); readln(a);
if a mod 2<>0 then writeln('Nhap lai');
until a mod 2=0;
readln;
end.