sql merge statement query

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SQL-Server

SQL-Server-2008

this is my query

MERGE StudentTotalMarks AS stm
USING (SELECT StudentID,StudentName FROM StudentDetails) AS sd
ON stm.StudentID = sd.StudentID
WHEN MATCHED AND stm.StudentMarks > 250 THEN DELETE
WHEN MATCHED THEN UPDATE SET stm.StudentMarks = stm.StudentMarks + 25
WHEN NOT MATCHED THEN
INSERT(StudentID,StudentMarks)
VALUES(sd.StudentID,25);
GO

but when i executing it showing error in line1 near as, so plz can any one remove this.
Thanks in Advance.