Answer:
n - k + 1
Step-by-step explanation:
This is assuming (because you did not say) that n and k are integers and n is greater than k.
Example: from 2 to 5 {2, 3, 4, 5} includes 5 - 2 + 1 = 4 numbers.
Example: from -6 to 4 includes 4 - (-6) + 1 = 11 numbers, namely {-6, -5, -4, -3, -2, -1, 0, 1, 2, 3, 4}
