SUMIF when checking two columns.

Morning,

Any help would be greatly appreciated, thanks...

I'm using: =SUMIF($I$2:$I$249,"s", AC$2:AC$249) to add up the value in AC if there is an S in column I.

I now want to amend this to only add up the values in column AC if there is an S in column I and a M in column B.

Thanks