Given an integer array nums of length n and an integer k, determine the maximum possible sum of a pair of elements nums[i] and nums[j] such that $0 \leq i < j < n$ and $j - i \geq k$.

Return the maximum pair sum that satisfies the condition.

Constraints:

• $2 \leq$ n $==$ nums.length $\leq 10^5$

• $1 \leq$ nums[i] $\leq 10^9$

• $1 \leq$ k $\leq$ n $- 1$

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