class GFG {

// Returns maximum sum in

// a subarray of size k.

static int maxSum(int arr[], int n, int k)

{

// n must be greater

if (n < k) {

System.out.println("Invalid");

return -1;

}

// Compute sum of first window of size k

int max_sum = 0;

for (int i = 0; i < k; i++)

max_sum += arr[i];

// Compute sums of remaining windows by

// removing first element of previous

// window and adding last element of

// current window.

int window_sum = max_sum;

for (int i = k; i < n; i++) {

window_sum += arr[i] - arr[i - k];

max_sum = Math.max(max_sum, window_sum);

}

return max_sum;

}

// Driver code

public static void main(String[] args)

{

int arr[] = { 1, 4, 2, 10, 2, 3, 1, 0, 20 };

int k = 4;

int n = arr.length;

System.out.println(maxSum(arr, n, k));

}

}