`C = (1 -` $\dfrac{1}{1234}$) `. (1 -` $\dfrac{1}{1234}$ `) . (1 -` $\dfrac{1}{1234}$ `) ... (1 -` $\dfrac{2021}{1234}$)
`= (1 -` $\dfrac{1}{1234}$) `. (1+1+1+....+1).(1 -` $\dfrac{2021}{1234}$)
`= (1+1+1+....+1) -` $\dfrac{1}{1234}$) `. (1+1+1+....+1).(1 -` $\dfrac{2021}{1234}$)
`=` $\dfrac{-1}{1234}$. $\dfrac{-787}{1234}$)
`=` $\dfrac{786}{1234²}$