When m is an integer, the solutions of the equations {x + y = m 5x + 3Y = 13 are all nonnegative?

When m is an integer, the solutions of the equations {x + y = m 5x + 3Y = 13 are all nonnegative?

X + y = m, multiply both sides of the equation by 3 at the same time to get 3x + 3Y = 3M, which is the same as the equation 5x + 3Y = 13, left minus left, right minus right
2X = 13-3m, so x = (13-3m) / 2, the value of X is brought into x + y = m, and y = (5m-13) / 2, because X and y are non negative integers, so (13-3m) / 2 ≥ 0, and (5m-13) / 2 ≥ 0, so 13 / 5 ≤ m ≤ 13 / 3, because m is an integer, so m = 3 or 4