A person leans over a cliff that is 1000 feet high and throws a rock vertically upward at a speed of 200 ft/sec. When it falls, it barely misses the cliff and falls on the ground at the base of the cliff. The height of the rock (in feet) above the ground t seconds after it was thrown is given by the function f(t) = −16t2 + 200t + 1000. Determine when the rock will hit the ground at the base of the cliff.