Question: What is the population of the city where KSTZ is located?

full_context: Document 1: Johnston, Iowa: Johnston is a city in Polk County, Iowa, United States. The population was 17,278 in the 2010 census, an increase of almost 100% from the 8,649 population in the 2000 census. It is part of the Des Moines–West Des Moines Metropolitan Statistical Area.
Document 2: KESZ: KESZ (99.9 FM; "KEZ") is an adult contemporary radio station based in Phoenix, Arizona. The station is owned and operated by iHeartMedia. Its studios are located in Phoenix near Sky Harbor Airport and its transmitter is in South Mountain Park.
Document 3: WQLH: WQLH (98.5 FM, "Star 98") is a Hot Adult Contemporary formatted radio station licensed to Green Bay, Wisconsin and serving Green Bay, Appleton, Oshkosh, and Northeast Wisconsin. The station is owned and operated by Cumulus Media. WQLH's studios are located on Victoria Street in Green Bay, while its transmitter is located near Suamico.
Document 4: Cedar Rapids, Iowa: As of the 2010 United States Census, the city population was 126,326. The estimated population of the three - county Metropolitan Statistical Area, which includes the nearby cities of Marion and Hiawatha, was 255,452 in 2008. Cedar Rapids is an economic hub of the state, located in the core of the Interstate 380.
Document 5: KPSZ: KPSZ (940 AM, "Praise 940") is a Christian radio station serving the Des Moines, Iowa, area. The station's studios are located on Locust Street in Des Moines along with Saga Communications' other Des Moines stations (KRNT, KSTZ, KIOA, KMYR and KAZR), while its 8-tower transmitter array is located near Hartford.
Document 6: KRBZ: KRBZ, 96.5 FM in Kansas City, is an alternative rock station. The station uses the brand name 96-5 The Buzz. The station is owned by Entercom Communications, with studios located in Mission, Kansas and a transmitter site in Kansas City's East Side.
Document 7: Adult contemporary music: Other popular names for the format include "Warm", "Sunny", "Bee" (or "B") and (particularly in Canada) "EZ Rock". The format can be seen as a more contemporary successor to and combination of the middle of the road (MOR), beautiful music, easy listening and soft rock formats. Many stations in the soft AC format capitalize on its appeal to office workers (many of them females aged 25–54, a key advertiser demographic), and brand themselves as stations "everyone at work can agree on" (KOST originated that phrase as a primary tagline, and other soft AC stations have followed suit).
Document 8: KRST: KRST (Nash FM 92.3 KRST) is a commercial radio station located in Albuquerque, New Mexico, broadcasting to the Albuquerque-Santa Fe, New Mexico area on 92.3 FM. KRST airs a country music format owned by Cumulus Media. Its studios are located in Downtown Albuquerque and the transmitter tower is located atop Sandia Crest east of the city.
Document 9: Saylorville, Iowa: Saylorville is a census-designated place (CDP) in Polk County, Iowa, United States. The population was 3,301 at the 2010 census. It is part of the Des Moines–West Des Moines Metropolitan Statistical Area. The Saydel Community School District is located in this area. The unincorporated community of Marquisville is in this area.
Document 10: Des Moines, Iowa: On September 22, 1851, Des Moines was incorporated as a city; the charter was approved by voters on October 18. In 1857, the name ``Fort Des Moines ''was shortened to`` Des Moines'', and it was designated as the second state capital, previously at Iowa City. Growth was slow during the Civil War period, but the city exploded in size and importance after a railroad link was completed in 1866.
Document 11: Des Moines, Iowa: Age spread: 24.8% under the age of 18, 10.6% from 18 to 24, 31.8% from 25 to 44, 20.4% from 45 to 64, and 12.4% who were 65 years of age or older. The median age was 34 years. For every 100 females, there were 93.8 males. For every 100 females age 18 and over, there were 90.5 males.
Document 12: KSTZ: KSTZ (102.5 FM, "Star 102.5") is a commercial FM radio station in Des Moines, Iowa. The station has a Hot Adult Contemporary radio format. KSTZ is part of Saga Communications' Des Moines Radio Group, along with KPSZ, KRNT, KAZR, KIOA, and KMYR, with studios located on Locust Street in Des Moines.
Document 13: WZST: WZST is an adult hits formatted broadcast radio station licensed to Westover, West Virginia, serving the Morgantown/Fairmont area. WZST is owned and operated by Laurel Highland Total Communications, Inc., through licensee LHTC Media of West Virginia, Inc.
Document 14: Des Moines metropolitan area: The Des Moines metropolitan area, officially known as the Des Moines -- West Des Moines, IA Metropolitan Statistical Area (MSA), consists of five counties in central Iowa, United States: Polk, Dallas, Warren, Madison, and Guthrie. The 2010 census population of these counties was 569,633, and the 2017 estimated population was 645,911.
Document 15: Middletown, Iowa: Middletown is a city in Des Moines County, Iowa, United States. The population was 318 at the 2010 census. It is part of the Burlington, IA–IL Micropolitan Statistical Area.
ground_truth: 645,911
sub_queries: What city is KSTZ located? Answer: Des Moines
what is the population of the Des Moines area Answer: 645,911