One day, Toki was researching numbers. While conducting his research, he discovered a new problem. He took a four-digit positive integer \( A \), where \( A=\overline{abcd} \) and \( a \leq b \leq c \leq d \). Again, \( B=\overline{dcba} \) and \( B-A=P \). The number obtained by writing the digits of \( P \) in reverse order is \( Q \). Determine the sum of all possible values of \( P+Q \).