Minimal Path Sum

Given a 2D square grid of positive integers, write a function to find the minimal sum of a path from the top left corner to the bottom right corner. You can only move to the right or down. Input will be a 2D array of integers, and output should be the minimal sum.

For instance, given the following grid:

[
  [1, 3, 4],
  [1, 5, 2],
  [4, 4, 1],
]

The minimal path sum is 10 (1 -> 1 -> 5 -> 2 -> 1).