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.