1 knife is 2.
Block,
1+ 1
=2;
The second knife is 4.
Block,
2+2
=
4;
The third knife is 7.
Block,
4+3
=7,
Because if the limit is cut vertically on a plane, it is impossible to cut four pieces, and at most it can only be cut like this.
The fourth knife is 13.
7+6
=
13,
. . .
Of course, if you can cut freely in space, it will be very simple.
2
,4
,8,
16, .....