Let the student's current average be denoted as ~S_{avg}~, and the current number of assignments they have completed to be ~N~. As we know that each assignment is weighted equally and is scored out of 100, we can deduce that the total points they have earned through all assignments so far is ~S_{avg}\cdot N~.

The student has one more assignment to complete, thus making the total points possible including the next assignment to be ~100(N+1)~.

In order to attain at least an ~80\%~ average, the student must obtain a total score of at least ~80(N+1)~. If this has already been achieved, simply print ~0~. Otherwise, print the missing score, which is ~80(N+1)-(S_{avg}\cdot N)~. If this missing score is greater than ~100~, print that it is impossible.

Final complexity: ~O(1)~