Newbie here. Would appreciate all your help.

I'm trying to write a formula for the following Scenario: Field Name - "Course Interest" has Certificate I, Certificate II, Certificate III, Certificate IV and Diploma in relevant cohorts e.g. Certificate IV in Engineering. I have to create a formula field (Course Level) that will output value as below: Certificate I - Level 1 Certificate II - Level 2 Certificate III - Level 3 Certificate IV- Level 4 Diploma - Level 5

I have the code as below but for Certificate I, II, III and IV it gives the same values for all.

IF
(LEFT(Course_Interest_c,13) = "Certificate I", 
    "Level 1", 

 IF(
        LEFT(Course_Interest_c,14) = "Certificate II", 
        "Level 2", 
     

  IF(
           LEFT(Course_Interest_c,15) = "Certificate III", 
            "Level 3", 

      IF(
                LEFT(Course_Interest_c,13) = "Certificate IV", 
                "Level 4",
             IF(
                    LEFT(Course_Interest_c,7) = "Diploma", 
                    "Level 5",
                    "Not Relevant"
                )
            )
        )))