I need to calculate payback period in R. I set it up like this:

...

library(FinancialMath)

initial_investment <- 63

annual_cash_flows <- c(-140,...-130) [all the cash flows are negative for 20 years)

payback_period <- initial_investment / annual_cash_flows[1]

payback_period: -0.4978229

...

Is the no -.49 alright? or is there something wrong with it? how can i interpret it please?