Get content of cell based on another cell in the same row?

I am totally lost with excel at the moment. Say I have these 3 rows:

idstring1 | 200

idstring1 | 250

idstring2 | 550

What I want to then do is calculate these numbers based on the first cell - idstring.

idstring1 = 200+250 = 开发者_开发百科450

idstring2 = 550

Can anybody help with this? The rows are in a different table also.

=sumif("A1:A3","idstring1","B1:B3")

That's assuming your strings are in the range A1:A3 and their corresponding values are in B1:B3