How to find the greater value in consective number in arrays javascript

Question

I would like to know how to find greater number in an array,

I have array arr and num denotes the consective number to be taken

in this case, 2+3, 5+6, 1+4

sumMethod([2,3,5,6,1,4],2)

in this case, 2+3+5, 6+2+4

sumMethod([2,3,5,6,2,4],3)

return the greater element in array.

function sumMethod(arr, num){
  for(var i=0;i<arr.length;i+num){
    for(var j=i;j<num;j++){
     //stuck 
   }
  }
}


Expected Output,
case1: 

11

case2:

12