Metallicities of RSGs compared with the radial metallicity gradient of Cepheids for the Galactocentric distance (R_GC). Filled red circles (top panel) and filled magenta squares (bottom panel) show the derived [Fe/H] of our target RSGs for VALD3 and MB99, respectively. Blue dots show [Fe/H] of Cepheids (Luck 2018) within 5 < R < 14 kpc as an indicator of the radial metallicity gradient of young stars. Blue dashed lines show the linear fit to the Cepheids’s metallicities. Open brown symbols show the weighted-mean metallicities and their corresponding standard errors of RSGs in star clusters or star-forming complexes within 6 ≲ R_GC ≲ 10 kpc measured by some previous works: Gazak et al. (2014) depicted with a diamond, Alonso-Santiago et al. (2017, 2018, 2019, 2020) and Negueruela et al. (2021) with triangles, and Fanelli et al. (2022) with a pentagon.