Hello, stuck on this one.

Everyday you travel through 3 traffic lights.

The probability of stopping at first traffic light is 0.6. If you stop at one set the prob you have to stop at the next is 0.9. If you dont have to stop at one set then the prob you dont have to stop at next is 0.7.

Find prob that you stop at exactly one set of lights.

I thought it was (0.6*0.1*0.7) + (0.4*0.3*0.9) + (0.4*0.7*0.3), which is wrong. Where is the mistake? Thanks