Erasing sheets for points.

We have $2^{2018}$ sheets of paper, with the number $1$ written on each of them. We perform the following operation. In every step we choose two distinct sheets; if the numbers on the two sheets are $a$ and $b$ , then we erase these numbers and write the number $a + b$ on both sheets. Then after how many steps the sum of the numbers on all the sheets is at least $2^{4036}$ .