Hi, I was told once that there is a formula for figuring out different kinda of combinations... I need to know the formula that will figure out the combination that fits into these rules...

I have a keypad, it has numbers from 1 - 9... I have to enter combinations of 4 numbers. The numbers in the combination always have to go from smallest to largest... So I could have 1579, but not 1845... The numbers can't repeat, so I can't have 1157...

What is the formula I would use to figure out how many combinations I could made?

Thank you in advance for any help.