program nhap;
uses crt;
var n:integer;
a,b:real;
begin
clrscr;
repeat
write('Nhap N: ');
readln(n);
if (n<=1) or (n>1000) then write('nhap lai!');
until (n>1) and (n<=1000);
repeat
write('Nhap a: '); readln(a);
write('Nhap b: '); readln(b);
if a>=b then write('a<=b. Nhap lai!');
until a<b;
readln
end.