Update query is not working Any idea?

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C#

SQL-Server

The updation is not happening when the start date and end date have with in the existed date range

How can i do this logic.I have the below query can any one correct it to achieve correct process

ALTER PROCEDURE [dbo].[spUpdateLeaveAllocation]
 (@ID int,
  @EmpCode int,  
  @StartDate Varchar(50),
  @EndDate Varchar(30),
  @NumDays float,
  @Reason Varchar(Max),
  @LeaveMode Varchar(50),
  @ApplyDate Varchar(50),
  @ReturnValue int out)
 As
 BEGIN
 Declare @CompCode int;
 SET @CompCode=(SELECT CompCode FROM EmpMaster WHERE EmpCode=@EmpCode);

 IF( SELECT COUNT(*) FROM LeaveAllocation WHERE (EmpCode=@EmpCode) AND (StartDate Between (@StartDate) AND (@EndDate)
OR @StartDate Between (StartDate) AND (EndDate) 
OR @EndDate Between (StartDate) AND (EndDate)
OR EndDate Between (@StartDate) AND (@EndDate)))>0
BEGIN
SET @ReturnValue=-2
END
ELSE
 BEGIN
 UPDATE LeaveAllocation SET StartDate=@StartDate,EndDate=@EndDate,[No.ofDays]=@NumDays,Reason=@Reason,LeaveMode=@LeaveMode,AppliedDate=@ApplyDate Where ID=@ID
 SET @ReturnValue=2
 END
 END