I'm doing a small program using processing and it's basically a map of europe and a question will ask where a certain capital is in europe. So what I want to do is for example if the question is where is Milan and the person say clicks on Madrid I want to use a gps to calculate the distance between Milan and Madrid. So hopefully the output would be "You have clicked on the wrong city you are xxx miles away from Milan"

Can someone please help me with code for this