Write a function named sumEveryNth that accepts an array of numbers, and a number n, and returns the sum of every nth number in the array. Use the Array.forEach() method.

Examples:

sumEveryNth([2, 3, 5, 7, 11, 13], 2) -> 23

sumEveryNth([2, 3, 5, 7, 11, 13], 3) -> 18