Conditional sum on cell values

[#1]

Good Morning to all,

I have a pivot table like this

Loading	Discharge		PodOriginale	Q.tà	Quan.+	Q.Tot	????
ABIDJAN	ANVERSA		ANVERSA	30	7	37	21
ABU DHABI	ABIDJAN		ABIDJAN	0	1	1
ALEXANDRIA	ABIDJAN		ABIDJAN	0	20	20

Now I need to have an additional column ('????') where the result must be the Sum of all the where "Discharge" is equal to the corrisponding "Loading" Port.

Referring to the example, I need, for the first row, an additional column where, I have the sum of the Abidjan of the second row.

In this simply case: 21

Is it possible?

Could anyone help me?

Thanks in advance