Bryn Mawr Bank Corp. said it agreed to buy Delaware County competitor First Keystone Financial Corp. in a stock and cash deal valued at $32.7 million based on Bryn Mawr's closing stock price yesterday of $16.32. The purchase, announced after the market closed today and expected to close in next year's third quarter, will add $500 million in assets and eight branches to the Bryn Mawr bank's operations. Bryn Mawr had $1.2 billion in assets at the end of September. First Keystone is based in Media.