Riddle of the week: Can this calculation work?

When looking for exciting puzzles, I come across some very old books. The following task comes from a book by Boris Kordemski, which appeared in 1959 in Moscow. A reader kindly sent me the German translation published in the GDR titled "Kopf, Kopf". Many thanks to Rolf-Dietrich Beran in Altlandsberg near Berlin!

The problem is briefly and dryly described in the book in one sentence:

How many plus signs do you have to put between the digits of the number 123456789, so that comes out as the sum of 99?

Short explanation: If you put a plus between all nine numbers, the sum is too small (1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 <99). If only between the 8 and the 9 no plus, the sum is too large (1 + 2 + 3 + 4 + 5 + 6 + 7 + 89> 99).