How can I store HUGE NUMBERS on unity? (256bit integer)

I tried using the BigInteger implementation for Unity, but it still overflows(Or maybe I used in a wrong way? Im not sure also, it can only do 20 characters of ints which is like 64 bits)... I need to store numbers as big as this:

105627842363267744400190144423808258002852957479547731009248450467191077417570

If this is possible using the BigInteger namespace implementation for Unity, can someone teach me how? thanks!