Puzzle - crossing bridge in minimum time

A good puzzle....crack it...

Four friends A, B, C, D
Need to cross a bridge..
A maximum of 2 peoples can cross at a time..
It is night and they have just 1 lamp.
People that cross the bridge must carry the lamp to see the way..
A pair must walk together at the speed of slower person...

Speeds of
A: 1 minute to cross bridge
B: 2 minutes to cross bridge
C: 7 minutes to cross bridge
D: 10 minutes to cross bridge

Now question is... "what is the total minimum time required by all 4
friends to cross the bridge....?













Answer is 17.