Aakash and Maths

Aakash is very good at maths. But his friend Arav doesn't think so. So to challenge him, he gives him a problem. He gives him an integer an $L$ and asks him to find the number of arrangements of integers $1$ to $L$ such that each arrangement satisfies the following property: Let the arrangement be: $A_1,A_2,....,A_L.$ The arrangement should be such that there exists a $R$ $(2 ≤ R ≤ L-1)$ such tha