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long solution(int n, long c, int d, int[] arr) {
        long left = 0, right = d, bestK = -1;

        while (left <= right) {
            long mid = (left + right) / 2;
            if (canAchieve(arr, d, c, mid)) {
                bestK = mid;
                left = mid + 1;
            } else {
                right = mid - 1;
            }
        }

        return bestK;
    }

    private static boolean canAchieve(int[] arr, long d, long c, long k) {
        int n = arr.length;
        long[] dp = new long[(int) (d + 1)];

        for (int i = 1; i <= d; i++) {
            dp[i] = dp[i - 1]; // Default: continue with max points without new exercise
            for (int j = 0; j < n; j++) {
                long previousDay = i - j - k - 1;
                if (previousDay >= 0) {
                    dp[i] = Math.max(dp[i], dp[(int) previousDay] + arr[j]); // Gain points from valid previous k slot
                }
            }
        }

        return dp[(int) d] >= c;
    }