Now very interesting and fadu puzzle

Now very interesting and fadu puzzle...   Please first think and then answer...
Its not as simple as you think. ...
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 d total minimum time required by all 4 friends to cross the bridge....?

Scratch ur brains...

Ans. 17 min 

A with B ( 2min ) and then A ( 1min ) back again with lamp..
then C with D ( 10min ) and then B ( 2min ) back again with lamp..
then A with B again ( 2min )