Write a function in python that calculates the payback period for a series of cash flows.

The function will take as input: A numpy array of cash flows. You can assume that element 0 is negative, representing the investment at t=0. You can also assume that all t>0 elements are positive, representing the cash flows at t=1 to t=N.

The function will return as output the payback period for the investment (the first year that the revenue is greater than or equal to the initial investment). If the project does not payback its initial investment, it should return -999.