Track the farthest reachable index (`max_reach`) and the current jump's end (`current_end`) while iterating through the array. Update `max_reach` at each step and increment the jump count when you reach `current_end`.

If `max_reach` doesn't advance beyond the current index, return `-1` (though the problem guarantees reachability, this helps debug edge cases).

Optimize by breaking early if `max_reach` already covers the last index, avoiding unnecessary iterations.