A computer system that includes a data-processing unit, a touch-sensitive, position-resolving input surface in data communication with the data-processing unit, and a freely movable input device, by which the input surface is to be touched for the purpose of inputting into the data-processing unit. The freely movable input device is a computer mouse, which is in data communication with the data-processing unit. With comparatively low sensor complexity, inter alia, both the absolute position of the computer mouse and also subtle position changes of the computer mouse can therefore be very easily detected by the data-processing unit.