Richard is buying new carpet for his office. The carpet comes in tiles which are 50 cm by 50cm. The office is 820cm long and 520cm wide. Since there is no pattern on the tiles, any piece of a given size can be used and orientation is not important. Richard wants to make sure that the whole tiles or pieces always meet in fours at their corners and that the smallest total number of pieces is used. How many tiles does Richard need to buy?