DistinctCount with filter.

I have the following data:

Table of Sellers

Saller_ID,Client_ID,Region
1,1,North
1,2,North
1,3,South
2,4,North
3,5,West
4,6,East
5,7,East
5,8,East
6,9,South
7,10,South
8,11,South

Table of Clients

Client_ID
1
2
3
4
5
6
7
8
9
10
11

I want to produce the following chart:

Region   Distinct Count of Client_ID

North             2

South            4

West             1

East              2

But I'm getting this as result:

Region   Distinct Count of Client_ID

North             3

South            4

West             1

East              3

 How can I distinct count ID by Region using a measure?