An insurance company models the amount N of loss (in thousands) if a homeowner has at least

Question:

An insurance company models the amount N of loss (in thousands) if a homeowner has at least one small claim. There is a constant k such that the mass of N is
P(N = n) = k / 2n + 1 for n = 1,2,3,4
with no other possible choices for N. Find the value of k to make this a probability mass function.