variable help

Hello, I've been working on an automated atm using computercraft and redpower. And I'm kind of stuck.
Basically what I have it do is accept three types of currency. $1, $5, and $10 tokens. I have a for loop written that will repeat a amount of times based on what the user
inputs when it asks them "how much money do you want to put into the account. I write it as
for i = 1,a do
This way the function can be repeated as many times as the user wants to put in money. Within the function I have an if statement that deals with recognizing the different types of currency. My problem comes when I want the system to store the information in a variable. Say for the account, playerd, I set the variable that stores the balance as playerd. My problem comes when I try and get the system to keep that information. I want the variable to add to itself depending on what currency is inserted. I use

a = io.read()
y = 0
for i = 1,a do
playerd = y + 1
end
print(playerd)
os.sleep(2)

however, this doesn't work because all it does is reset the variable, so the variable playerd always equals 1.

You don't need to use two variables, just use 1 and add it to itself. "y = y + 1"

When using read() and you want to use that variables as a number, you should convrrt it to a number with tonumbrr(read()) for loops

why not just

a=read()
y=0
playerd=y+a
print(playerd)
sleep(2)

How is that different from:

print(read())
sleep(2)

I think that I need a better description of how this "currency insertion" process works. That's what's missing for me. As it is, the OP code (with some correction for apparent intention) could be replaced by:

playerd = playerd + read()
print(playerd)
sleep(2)

But this ignores the step where the "inserted currency" is evaluated and accepted/rejected/whatever.

I'm aware that this function may not exist yet, but having a placeholder could help with understanding the appropriate structure to use.