4. On the board, the numbers $1 / 1, 1 / 2, 1 / 3, 1 / 4, \ldots 1 / 100$ are written.

It is allowed to erase any two numbers a, b and write $a b + a + b$ in their place,

then do the same with any two of the remaining numbers, and so on. What number can be left last?