Nationwide, the Case-Shiller index — which tracks prices in 20 U.S. metro areas — had its fourth consecutive month without annual price gains in any city in July. However, the index did report a slowdown in the rate of home price decline. It showed a 2.2 percent drop in home values from May to July this year, which is less than half the decline seen from February to April. The index only covers single-family homes, not including co-ops or condos. The average asking price of a Park Slope brownstone since January is down to $2.35 million, from $2.8 million.