An explosion breaks a rock into three parts in a horizontal plane. Two of them go off at right angles to each other. The first part of mass $1\,\text{kg}$ moves with a speed of $12\,\text{ms}^{-1}$ and the second part of mass $2\,\text{kg}$ moves with $8\,\text{ms}^{-1}$ speed. If the third part flies off with $4\,\text{ms}^{-1}$ speed, then its mass is