main unsigned x1; int b=-1; x1=b; printfux1;

The output of this program is likely to be the maximum value representable by an unsigned integer on the system, which is 4294967295 on most systems with 32-bit unsigned integers.

The reason for this is that when the signed integer variable b is assigned the value of -1, it is represented in binary as all 1's (in a two's complement representation). When this value is then assigned to the unsigned integer variable x1, the value is interpreted as an unsigned integer, and the resulting value is the maximum value that can be represented by an unsigned integer with the same number of bits.

The printf statement then prints this value as an unsigned decimal integer, resulting in the output of 4294967295 (assuming a 32-bit unsigned integer)