15 - 7 = 8
Where the 7 is the sort where the top-left vertical is present.
Move the two vertical matches of the + to make a 1 to the left of the 5. And move the 4's one horizontal upwards to make the 7's top horizontal. So I move 3 matches.
Visualization:
. +--------- A | | +---------+
. | A | | | |
. | A | | ----------- | |
. +--------+ -----+----- +CCCCCCCC+ +---------+
. | B | ----------- | |
. | B | | |
. ---------+ B | +---------+
Becomes:
. A +--------- +CCCCCCCC+ +---------+
. A | | | | |
. A | | | ----------- | |
. + +--------+ ----------- | + +---------+
. B | | ----------- | |
. B | | | |
. B ---------+ | +---------+
