Four people want to cross a dangerous bridge in dark. They have only one flash light. The bridge can hold up to two persons at a time. People walking on the bridge must use the flash light to avoid drop off the bridge. The highest walking speed for each person is different. It takes at least 2, 3, 7, 10 minutes respectively for each person to cross the bridge. What is the minimum time (in minutes) for the group of four to cross the bridge safely?