smc

Call a $7$-digit telephone number $d_1{d}_2{d}_3-d_4{d}_5{d}_6{d}_7$ memorable if the prefix sequence $d_1{d}_2{d}_3$ is exactly the same as either of the sequences $d_4{d}_5{d}_6$ or $d_5{d}_6{d}_7$ (possibly both). Assuming that each $d_i$ can be any of the ten decimal digits $0,1,2,\dots ,9$, the number of different memorable telephone numbers is