Cristiano Ronaldo writes a sequence $a_1$, $a_2$$,...,$ $a_{100}$ of integers in which the first and last terms are equal to $0$. Except for the first and last terms, each term $a_i$ is larger than the average of its neighbours $a_{i−1}$ and $a_{i+1}$.

What is the smallest possible value for the term $a_{19}$?