The nine digits 1 through 9 are written out in a row in ascending order as shown in the upper row of the illustration. The object is to insert between the digits, without changing their positions in the row, several arithmetical signs so that to get exactly a hundred as the result. From the four arithmetical signs you can use only plus (+) and minus (-). The solution with six signs is shown as an example in the lower right corner of the illustration. But can you find a solution where three signs are used only?