Consider randomly selecting n segments of pipe and determining the corrosion loss (mm) in the wall thickness for each one. Denote these corrosion losses by Y1, ... , Yn. The article "A Probabilistic Model for a Gas Explosion Due to Leakages in the Grey Cast Iron Gas Mains" (Reliability Engr. and System Safety (2013:270-279) proposes a linear corrosion model: Yi = tiR, where ti is the age of the pipe and R, the corrosion rate, is exponentially distRibuted with parameter l. Obtain the maximum likelihood estimator of the exponential parameter (the resulting mle appears in the cited article).