Harry is a postman. He's got a post office with a size of n*m(a matrix / 2D array). Each slot at the 2D array represents the number of letters in that spot. Harry can only go right and down. He starts at (0, 0), and ends at (n-1, m-1). n represents the height, and m the length. Return the maximum amount of letters he can pick up. He can only pick up letters if he is on that spot.
harry([[5, 2], [5, 2]]) ➞ 12
// (5+5+2)
harry([
[1, 2, 3, 4, 5],
[6, 7, 8, 9, 10],
[11, 12, 13, 14, 15]
]) ➞ 72
// (1+6+11+12+13+14+15)
harry([[]]) ➞ -1
If the matrix is empty, return -1.