HELP with IF function

Hello,

So I have a problem with my file. I need to use IF function, but I don't understand. The main thing is that I need to calculate the days that the debt has not been paid ( For example: today is 2018.11.04, but you need to pay that amount of money on 2018.11.01. That means, 3 days of debt.) . But if the amount paid up to the specified date, the excel field must display "-".

So, I need the function with IF.
Link of photo - https://www.part.lt/img/fb5ac78912f343325a0f9676e04df027190.PNG


I'll be waiting for your response.