Excel Converting Zip plus 4 to just 5 number zip

Hi

Using Excel 2007

I have a column of zip codes that are zip plus 4.  I need a formula that will change it to just the regular zip.

Zip Code
93309-2156
93932-0446
92509-3600
93711-5770

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The data can be numbers or text.  

I would like the result to be as follows.

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Zip Code
93309
93932
92509

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Thanks!

Rowby