1. An apparatus for receiving a key input, the apparatus comprising at least one processor to implement: a display unit configured to display a keypad to receive the key input; a detection unit configured to detect a click gesture using a length from an arbitrary point on a user's hand to a fingertip; and a control unit configured to control the display unit to display a result of the key input according to the click gesture detected on the keypad, wherein the detection unit detects the click gesture based on a first ratio of the fingertip, the first ratio being of the length from the arbitrary point to the fingertip before the click gesture is made to the length from the arbitrary point to the fingertip when the click gesture is made, wherein the detection unit detects the click gesture further based on second ratios of two or more fingertips, the second ratios being based on lengths from the arbitrary point to the two or more fingertips before and when the click gesture is made, and wherein the detection unit detects the click gesture in response to a difference between the first ratio of the fingertip and an average of the second ratios of the two or more fingertips being greater than a threshold.