Stupid Machine

As usual, I went to work in the morning. Unfortunately, I found out that my manager bought a new machine and I have to learn to operate it. There are $N$ boxes in a line (numbered $1$ through $N$). Initially, the boxes are empty, and I need to use the machine to put tokens in them. For each valid $i$, the $i$-th box has a maximum capacity $S_i$ tokens. I can perform the following operation any