M&T; Bank Corp. agreed to buy Keystone Financial Inc. for $1 billion to expand in neighboring Pennsylvania, where it'll become the state's fifth-largest bank. The combined company will become the 29th largest U.S. bank with assets of $29.8 billion, 450 branches and about 1,500 automated teller machines in New York, Pennsylvania, Maryland and Virginia. Buffalo, N.Y.-based M&T; will pay 0.05 shares or $21.50 cash for each Keystone share. Keystone shares jumped $4.25, or about 27%, to close at $20 on Nasdaq. M&T; fell $14.06 to close at $400.94 on the NYSE. M&T;, with assets of $22.8 billion, is more than three times larger than Harrisburg, Penn.-based Keystone, which has assets of $7 billion.