I would like to create a formula that a) compares four columns of data (see below)

Sep              Oct
name    units    name    units
apple   2        apple   3
pear    3        pear    7
orange  4        banana  6
banana  3        toffee  5

then b) merges the two "names" column into one column, dropping any duplicates but still retaining the two unit columns (for months Sep and Oct)

         Sep      Oct
name    units    units
apple    2         3
pear     3         7
orange   4         0
banana   3         6
toffee   0         6

then c) creates a third column that compares "Sep units" against "Oct units" and produces the total in the "difference" column

         Sep      Oct
name    units    units     difference
apple    2         3            1
pear     3         7            4
orange   4         0           -4
banana   3         6            3
toffee   0         6            6