Simple java solution by cumulative sum, beating 96.78%

```
public class Solution {
public double findMaxAverage(int[] nums, int k) {
if(nums.length < k){
return -1;
}
for(int i = 1 ; i < nums.length ; i ++) {
nums[i] = nums[i] + nums[i - 1];
}
double max = nums[k - 1];
int j = 0;
for(int i = k ; i <nums.length ; i ++) {
if(max < (nums[i] - nums[j])) {
max = nums[i] - nums[j] ;
}
j ++;
}
return max / k;
}
}
```