You are given three grids, where each of them is 5 by 5 in size.
Grids have empty cells marked with "▢" and filled cells marked with "X".
Your task is to generate a referring expression that best describes the target grid while distinguishing it from the two other distractor grids.
The first grid is the target grid, and the following two grids are the distractors.

Target grid:

▢ ▢ ▢ ▢ X
▢ ▢ ▢ X ▢
▢ ▢ X ▢ ▢
▢ X ▢ ▢ ▢
X ▢ ▢ ▢ ▢

Distractor grid 1:

X ▢ ▢ ▢ X
▢ X ▢ X ▢
▢ ▢ X ▢ ▢
▢ X ▢ X ▢
X ▢ ▢ ▢ X

Distractor grid 2:

X ▢ ▢ ▢ ▢
▢ X ▢ ▢ ▢
▢ ▢ X ▢ ▢
▢ ▢ ▢ X ▢
▢ ▢ ▢ ▢ X

Instruction: Describe the target grid.
Generate the referring expression starting with the tag "Expression: " for the given target grid. Omit any other text.
Expression: In the target grid, the top left corner has three blank cells followed by a filled cell, while the first column has two blank cells at the top and the bottom. The middle row has two filled cells.
assistant
Expression: The grid with the "X" starting from the bottom left corner and moving diagonally upwards to the right, filling the cells in a staircase pattern.
assistant