You are given a $0$ indexed, strictly increasing integer array nums and a positive integer diff.

A triplet of indices (i, j, k) is defined as an arithmetic triplet if the following conditions are satisfied:

• i $<$ j $<$ k

• nums[j] $-$ nums[i] $=$ diff

• nums[k] $-$ nums[j] $=$ diff

Return the total number of unique arithmetic triplets.

Constraints:

• $3 \leq$ nums.length $\leq 200$

• $0 \leq$ nums[i] $\leq 200$

• $1 \leq$ diff $\leq 50$

• nums is strictly increasing.

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