Can anyone help I have a program to write

I need to write a pgm that converts a series of input temperatures in either degree Celsius or Fahrenheit and calculate the absolute temperature in degrees Kelvin.
So if the input was 32F the result would be 273K.Repeat this process for any number of input temperatures and scales.
The pgm should finish with the following message "Error - Negative Absolute Value do not assume the input will only be 'C' OR 'F'.
The required formulae are
K = C + 273 or K = (F-32) / 1.8 + 273

Can anyone help! I'me slowly going insane......