1081
-0432
Do each of the columns one by one, from right to left: units/ones, tens, hundreds, thousands, (and ten thousands, hundred thousands...): 1, 10, 100, 1000, (and 10 000, 100 000...).
Do units first: "1 - 2" Wait! you can't subtract 2 from 1! Oh Yeah, you have some stuff in the tens column to use (80). Take 10 from there, leaving 70, and make your 1 + 10 = 11. 11-2=9 (unit number is 9)
Do tens next: "80 - 30" Wait! I took 10 off the 80, so it's now "70-30" = 40 (tens number is 4)
Do hundreds: "000 - 400" Wait! I can't subtract 400 from 0! Oh yeah, let's see if I can borrow something from the thousands column. 1000-400 = 600 (hundreds number is 6)
Do thousands: "1000-0000" Waitaminit! I already borrowed 1000 for the hundreds thingy, so my thousands column is "0000-0000" = 0 (thousands number is 0)
So,
1081
-0432
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0649 --> 649