If you get 1 and 2, then you have to exclude 7.

Similarly, 2 and 3, 3 and 4, and so on.

So we have to rule out eight such situations.

That's 8*7=56.

But when we calculate 1 and 2 plus 3, we repeat 2 and 3 plus 1.

There are seven similar situations, 123, 234, 345, 456, 567, 678, 789,

So the final result is C93-8*7+7=35.