program HelloWorld
    include 'mpif.h'
    integer my_rank
    integer p
    integer source
    integer dest
    integer tag
    character*100 text, message
    character*10 digit_string
    integer size
    integer status(MPI_STATUS_SIZE)
    integer ierr      
    
    call MPI_Init(ierr)

    call MPI_Comm_rank(MPI_COMM_WORLD, my_rank, ierr)
    call MPI_Comm_size(MPI_COMM_WORLD, p, ierr)

    tag = 0
    source = my_rank - 1
    if ( source < 0 ) source = p - 1
    dest = my_rank + 1
    if ( dest >= p ) dest = 0
    
    if (my_rank  ==  0) then
        text = "Hello world from process "
        call MPI_Send(text, len_trim(text), MPI_CHARACTER, dest, tag, MPI_COMM_WORLD, ierr)
    end if
    call MPI_Recv(message, 100, MPI_CHARACTER, source, tag, MPI_COMM_WORLD, status, ierr)
    if (my_rank  /=  0) then
        call MPI_Send(message, len_trim(message), MPI_CHARACTER, dest, tag, MPI_COMM_WORLD, ierr)
    end if

    write(6,FMT="(A,I)") message,my_rank

    call MPI_Finalize(ierr)
end program HelloWorld