function maxSumArr(arr, num) {
let maxSum = 0;
let tempSum = 0;
if(arr.length < num) return null;
for(let i = 0; i < num; i++) {
tempSum += arr[i];
}
tempSum = maxSum;
for(let i = num; i < arr.length; i++) {
tempSum = tempSum - arr[i - num] + arr[i];
maxSum = Math.max(tempSum, maxSum);
}
return maxSum;
}