A lift ascends with constant acceleration $a=1 \mathrm{m} {\mathrm{s}}^{-2}$ , then with constant velocity and finally, it stops under constant retardation $a=1 \mathrm{m} {\mathrm{s}}^{-2}$ . If total distance ascended by the lift is $7 \mathrm{m}$, in a total time of the journey is $8 \mathrm{s}$. Find the time (in second) for which lift moves with constant velocity.