A flexible pavement has a structural number $($SN$)$ of $4\mathrm{.}0.$ The initial and terminal serviceability indices are $4$ and $2\mathrm{.}5,$ respectively. The soil has a CBR of $8\mathrm{.}0.$ The overall standard deviation of traffic is $0\mathrm{.}45$ and the reliability level is $90\%.$ The pavement is designed for $2,500$ equivalent $18$ kip single$-$axles per day. Using the flexible pavement design chart below, what is the design life cycle of the pavement? $($i$.$e$.,N=?)$