'Calculate the exponentials of big negative value

I would like to know how can I get the exponential of big negative number in R? For example when I try :

   > exp(-6400)
   [1] 0
   > exp(-1200)
   [1] 0
   > exp(-2000)
   [1] 0

but I need the value of above expression, even if it is so small, how can I get it in R?

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