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#include <iostream>
#include <vector>
#include <cmath>

class Solution {
public:
    long long maxSubarraySum(std::vector<int>& nums, int k) {
        int n = nums.size();
        std::vector<long long> dp(n);
        long long diff = 0;
        for (int i = 0; i < k; ++i) {
            diff += nums[i];
        }
        long long max_sum = dp[k - 1] = diff;
        
        for (int i = k; i < n; ++i) {
            diff += nums[i] - nums[i - k];
            dp[i] = std::max(dp[i - k], 0ll) + diff;
            max_sum = std::max(max_sum, dp[i]);
        }
        
        return max_sum;
    }
};