Usa la formula di Wallis per trovare il valore approssimativo di pi.
Use the following "Wallis's formula" to find the the approximate value of pi.
π = 2・{(2・2・4・4・6・6・8・8・・・)/(1・3・3・5・5・7・7・9・・・)}
Let's observe how the approximate value of pi can be obtained as the number of numerator and denominator numbers increases.