4 people want to cross a bridge at night, with just one lamp, and the bridge can take at most
A group of two walks at the slowest speed of the two.
A group or a single individual is allowed to walk only if it carries the lamp.
Albert can cross the bridge in 1 mn
Bernard in 2 mn
Charles in 4 mn
And Denis in 8 mn
How can they manage to cross the bridge in the minimum time ?