Thank you all.
I confirm that the button is wired correctly (it was already tested with the led). Tested with the diagonal and, separately, the ohm meter.
@hammy: my circuit is the Button tutorial.
@MarkT : when the button is not pressed, the voltage between the input pin and the ground is 0V. When it is pressed, it is +5V. Seems normal...
P.S.: one more point, the Blink tutorial works as expected.