When i try to open my Oracle 9i database i get this :

ORA-01122: database file 2 failed verification check
ORA-01110: data file 2: 'D:\ORACLE\ORADATA\ORCL\UNDOTBS01.DBF'
ORA-01200: actual file size of 39680 is smaller than correct size 40960 blocks

How can i correct this i cant open my database.

thanks