Given two integer arrays nums1 and nums2, each of length n, and an integer diff, determine the number of index pairs $(i, j)$ that satisfy the following conditions:

• $0 \leq i < j < n$

• nums1[i] - nums1[j] $\leq$ nums2[i] - nums2[j] + diff

Return the total count of such valid pairs.

Constraints:

• n == nums1.length == nums2.length

• $2 \leq$ n $\leq 10^5$

• $-10^4 \leq$ nums1[i], nums2[i] $\leq 10^4$

• $-10^4 \leq$ diff $\leq 10^4$

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