Using IF else statements in stored procedure

I used two databases, student database and PIN database. Am trying to create a login verification where by if PIN is used with student's reg-number, the stored procedure query, checks if user THE USER REG-NUMBER INPUT is valid in the student database.

It also checks if PIN input is valid in the PIN database. If true, it then updates the PIN database in the column of used PIN number, with other credentials as entered by the user on first time usage of PIN number.

And if user tries to use same PIN again the query checks if credentials are right else prompts wrong PIN, but if credentials are correct pin usage adds up by 1 and user can gain access. #pls I limited pin usage count by 4 times alone# here below is my procedure

pls below is my stored procedure as managed to fix up;

CREATE PROCEDURE [dbo].PinValidation     
@Pin_no Nvarchar(24), 
@Reg_Num Nvarchar(24), 
@Session Nvarchar(50), 
@Program Nvarchar(50) 
AS 
BEGIN 
SET NOCOUNT ON; 
DECLARE @Tries INT= 0 
IF EXISTS (SELECT PIN.Reg_Num 
FROM 
student 
INNER JOIN 
PIN 
ON student.Id = PIN.Id                
WHERE (student.Reg_Number = @Reg_Num) 
AND (PIN.Reg_Num = @Reg_Num)) 
BEGIN 
UPDATE [PIN] SET [Tries]= Tries +1 
WHERE Reg_Num = @Reg_Num            
AND Pin_no = @Pin_no AND p_Session = @Session AND Program = @Program  -- increases the value of tries by 1 each time pin is used 
SELECT [Tries] 
FROM PIN 
WHERE Pin_no = @Pin_no     
END 
ELSE 
BEGIN 
UPDATE [PIN] 
SET Reg_Num = @Reg_Num,
 p_Session = @Session,
 Program = @Program,
 Tries = @Tries + 1 
WHERE [Pin_no] = @Pin_no  --INSERTS/UPDATES PIN values if pin_no=@Pin_no and the above INNER JOIN END IF @Tries IS NOT NULL BEGIN 
IF EXISTS (SELECT Pin_no FROM PIN                     
WHERE Pin_no = @Pin_no) 
BEGIN 
DELETE FROM [PIN] 
WHERE [Tries] = 4 --Check the Amount of times pin has been used if >3 delete entire pin row 
END 
END 
ELSE 
BEGIN SELECT -1 
END 
END