Graph Algorithms · Superchallenge problem

A directed weighted graph has vertices $s,a,b,c,d,t$ and edges $s\to a{:}2$, $a\to b{:}-5$, $b\to a{:}-1$, $s\to c{:}4$, $c\to d{:}3$, $d\to t{:}2$, $c\to t{:}9$. Vertices $a$ and $b$ have no outgoing connections except to each other. What is the shortest path length from $s$ to $t$?