Trying to retire at 50

Hi, I work at a federal agency and have about 15 years of federal experience. Trying to retire at 50. Is it possible with the State Department? My question is whether the 20 years creditable service has to be with State Department? Or else could I just apply to work there at 49 and then retire in like a year at 50 from State Department with 20 years total government experience? Trying to see if I could game the system or if the 20 years must be in the Foreign Service? TIA!