Write some Software -- 2

i have a website made in magneto platform. in my website there is a whole inventory option but i want separate quantity option for each size or color. suppose i have a t-shirt having 3 sizes ( small :10 pieces- medium :8 pieces and large 6 pieces ) . i want when i get order of small one the quantity reduce automatically from whole and separate inventory. only those people contact who answer this question : 2+2= ?