For loops and while do in pascal

i have written the following program

program test;
var alt, dis, answer : integer;
var sum : real;
begin

writeln ('would you like to fly at a specific height? 1 for yes 2 for no');
readln (answer);
if (answer = 1) then
begin
writeln ('enter chosen cruising level');
readln (alt);
writeln ('enter flight length');
readln (dis);
sum := (((750.44-(0.013717*alt))/60)*((dis/5)-((alt/1000)*2)))+(((750.44-(0.013717*(alt/2)))/60)*((alt/1000)*1.4))+(((750.44-(0.013717*(alt/2)))/60)*((alt/1000)*0.9));
writeln ('the total fuel consumption for this flight is, sum :4:2 , kgs')
end
else;
begin
writeln ('enter distance of flight');
readln (dis);

This is as far as i've got with my program. i now need the program to work out the optimum cruising level for the distance entered. i know i will have to use a for loop or while do command but don't know how they work properly.
i need to let the end user enter the distance they want to travel and then for the program to test the fuel consumption for each height upto 30,000ft for that distance and then display the best result (the one that uses the least amount of fuel).
The range is between 0 - 30,000ft in intervals of 1,000.

if you could let me know how to finsh this program or how to improve it i would be grateful.

thanx
scoobydoobydrew