Summing commission totals by month.

Need help creating an Excel formula for adding commission income depending on who the salesperson is and the month in which the commission was received.

A B C D
1 MONTH COMMISSION MONTH TOTALS
2 Jan $1,000 January
3 Feb $2,000 February
4 Jan $100 March
5 Feb $100 April
6 Mar $2,000 May
7 Jan $400 June


In the cells above is a sample of what the sheet looks like. I like to figure out this:
• If cell A2 says “Jan” then take the dollars amount adjacent at B2 and put it in cell C2 (total Jan Income)
• If cell A4 says “Jan” again, then take the dollar amount adjacent at B4 and add it to the amount that is already at C2 (total income for Jan)

I’ve tried using “If then” formulas, but just can’t see to get it right.

Can anyone help?

Albert Allegue