$1$st step: Each process identifies primes up to $\sqrt[\phantom{2}]{n}$

$2$nd step: each process sieves the list of integers with $1/$pth of the primes between $2$ and $\sqrt[\phantom{2}]{n}$

$3$rd step: Processes OR$-$reduce their arrays into a single array held by process $0$

Last step: Process $0$ counts unmarked elements and prints prime number count

write a MPI code for this logic