Let $s$ be the set of complex numbers of the form $a + bi,$ where $a$ and $b$ are integers. we say that $z \in s$ is a unit if there exists a $w \in s$ such that $zw = 1.$ find the number of units in $s.$

When patey pontoons issued 4% bonds on january 1, 2018, with a face amount of $660,000, the market yield for bonds of similar risk and maturity was 5%. the bonds mature december 31, 2021 (4 years). interest is paid semiannually on june 30 and december 31?