The inventory of houses for sale dropped in February for the ninth month in a row, and pending contracts indicate that sales should be strong this year, the Des Moines Area Association of Realtors said. The number of contracts written last month was 994, a 22 percent jump from February 2012. There were 3,760 active listings, which is down from the same period last year by more than 500 listings. The median sales price increased more than $30,000 over the same time last year.