Casting a hex number under unchecked scope

Well I am trying to understand how negative numbers work and trying to cast a hex number as we can see below to a singed integer, but the question is why does this work only under "unchecked" scope?

 unchecked
 {
     signedInt = (int)0xffffffff; // -1 (1111........ 1111)
     signedInt = (int)0xfffffffd; // -3 (1111........ 1101)

 }