C++: Need help with gas temp program

Write a program named p_26.cpp that accepts a value of Vm and checks that it is between 12 and 18. The program should return the gas temperature in degrees Celsius when Vm is between 12 and 18 and an error message when it isn't. There are several testcases that must all work. You will only need to input the voltage, but not actually type it since the testcases have the input pre-recorded