Study the table below, which shows the mean annual rainfall recorded at six selected meteorological stations across the Sahel region of Africa.
| Meteorological Station | Dakar (Senegal) | Nouakchott (Mauritania) | Tombouctou (Mali) | Niamey (Niger) | N'Djamena (Chad) | El Fasher (Sudan) |
|---|---|---|---|---|---|---|
| Mean Annual Rainfall (mm) | 495.5 | 158.0 | 182.5 | 541.0 | 510.5 | 266.5 |
Calculate the overall mean annual rainfall for these six stations.