Avoid integer overflow in distance calculation
Avoid integer overflow in distance calculation

Fixes #3, without using a poor distance calculation.
wowza512/s,dwfreed/pebble-caltrain,dwfreed/pebble-caltrain,ashneo76/pebble-caltrain,dwfreed/pebble-caltrain,wowza512/s,edmundnoble/pebble-caltrain,dwfreed/pebble-caltrain,Katharine/pebble-caltrain,wowza512/s,Katharine/pebble-caltrain,edmundnoble/pebble-caltrain,ashneo76/pebble-caltrain,edmundnoble/pebble-caltrain,Katharine/pebble-caltrain,wowza512/s,ashneo76/pebble-caltrain,Katharine/pebble-caltrain,edmundnoble/pebble-caltrain,ashneo76/pebble-caltrain