s 2
t 2
( s 1 , s 3 )
(1, 1)
(4, 2)
( t 1 , s 3 )
(6, 3)
(7, 7)
( s 1 , t 3 )
(7, 4)
(2, 1)
( t 1 , t 3 )
(9, 3)
(7, 1)