In data, there are so some cases that pi is greater than the length of sequence until i-th pair to be processed..

like this..

0 m
0 r
0 l
1 1
2 2
1 1
0 z
0 v
3 2
0 p
0 y
0 p
5 5
0 c
12 12
6 4
21 6
0 j
0 y
0 n
40 34 <-

..what's that?