With two's complement representation the result of addition is correct if the carry into the high order column is the same as the carry out of the high order column. The carry can be one or zero. Overflow is detected by comparing two bits, an easy thing to do with electronics. Here are some more examples:

11111 111
 0011 1111  ( 6310)
 1101 0101  (-4310)
 0001 0100  ( 2010)

00000 011
 1100 0001  ( -6310)
 0010 1011  (  4310)
 1110 1100  ( -2010)

01111 100
 0011 1111  (  6310)
 0110 0100  ( 10010)
 1010 0011  ( -9310)

10000 000
 1100 0001  ( -6310)
 1001 1100  (-10010)
 0101 1101  (  9310)

high column이 맨앞의 부호비트를 말하는거죠?