Need help understanding degrees of unsaturation

[RidingMyBike]

When I did this problem I counted carbons and hydrogens etc. and used this variant of the formula
#C + #N/2 + 1 - #H/2 - #X/2

The answer was 13, and it explained I can count 3 rings + 10 double bonds = 13 degrees of unsaturation. I will be using this method on a question like this, but somehow I can't get the answer 13 when I try using the formula? I keep getting 12. What went wrong . . .

20 carbons, 17 hydrogens, 1 halogen right?

20 + 1 - 17/2 - 1/2
20 + 1 - 18/2
20 + 1 - 9
20 - 8
= 12 ??

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[DAT Destroyer]

When I did this problem I counted carbons and hydrogens etc. and used this variant of the formula
#C + #N/2 + 1 - #H/2 - #X/2

The answer was 13, and it explained I can count 3 rings + 10 double bonds = 13 degrees of unsaturation. I will be using this method on a question like this, but somehow I can't get the answer 13 when I try using the formula? I keep getting 12. What went wrong . . .

20 carbons, 17 hydrogens, 1 halogen right?

20 + 1 - 17/2 - 1/2
20 + 1 - 18/2
20 + 1 - 9
20 - 8
= 12 ??

You do not need to plug in the formula.
Just remember that;
Double bond is 1 degree of unsaturation
Ring is 1 degree of unsaturation
Triple bond is 2 degrees of unsaturation

Just count, you will get the correct number.

Hope this helps