Step 1: Is it a city?Yes: "à" No: continueStep 2: Ends with an -e?
Yes: "en" (féminin singular). No: continue
Step 3: Ends with an -s?
Yes: "aux" (Plural. ) No: continue
Step 4: Is it a country and it doesn't end with an -s or an -e?
Then it is "au" (male singular)