Fitting Block Puzzle

This game's goal is to fill and fit various block shapes.

block puzzle's features
1: filling and fitting simple 4x4 5x5 6x6 center gray rectangle with square block shapes.
2: all block shapes are unique
3: no rotate of blocks is allowed.
4: very interesting.

because all block shapes are unique, your brain will be trained into clever brain.

difficulties consist of three categories (easy, normal, hard)

easy difficulty is 4x4 and filling and fitting simple center 4x4 gray rectangle with 5 block shapes.
normal difficulty is 5x5 and filling and fitting simple 5x5 gray rectangle with 7 block shapes.
hard difficulty is 6x6 and filling and fitting simple center 6x6 gray rectangle with 9 block shapes.

each difficulty has 475 levels and all 1425 levels totally.