A computer system including an electronic device and an input device is disclosed. The electronic device includes a touch display area and a control unit. The input device includes a plurality of positioning structures and a grounding piece. The positioning structures are electrically connected to the grounding piece. When the input device is disposed on the touch display area of the electronic device, the control unit detects positions of the positioning structures on the touch display area, calculates a coverage area covered by the input device on the touch display area according to the positions, and determines a range of a display area of the touch display area according to the coverage area.