As you may know, capital gain or loss on the sale is determined by computing the difference between the sale price and your adjusted tax basis on shares sold.
Since you got the shares as an employee, there are several possibilities depending on the type of plan that was offered to employees. Sometimes the employees are able to buy shares for less than the full market price or are granted options. Other times the value of the stock is included in your compensation so your basis is the price on the day you acquired it.
There are links at the Wrigley site for investors with stock price information that can be found on the Historical Price Lookup page if you know the dates and number of shares you purchased.
Without many more details no one but the company will be able to tell you your basis in the stock.
Contact information for Wrigley can be found at http://www.wrigley.com/wrigley/contact_us.asp
I hope this helps even though a definite answer can not be given from what you know.
From your description you can likely use the full price on the date of purchase. Obviously the share price was different on different dates. Sorry, but without dates and number of shares there is no way to determine your cost basis in the shares.
If you can find out how many shares you bought on what dates the cost basis can be computed.
There is simply no way to compute your basis without more information, so I tried to help you with some possible sources to contact for more information and a tool to find the price on the dates you determine that you bought shares.
The default rule if you can not find a cost is to use a zero cost on the sale so it will be better for you if you can obtain the information needed to find the cost basis.