Formula to work backwards from repayments to find capital

I am looking for a formula that will give me the amount borrowed when the monthly repayment is know.

So the variables that I know are the monthly repayment, the length of the loan (in either months or years) and the interest rate.

I want to be able to put the monthly repayment in say cell A1, the length of loan (in months) in A2, and the interest rate % in A3. From this information I want to know how much the initial amount borrowed will be.

Can anyone help please?