Graph Algorithms · Challenge problem

A directed weighted graph has vertices $s,a,b,t$ and edges $s\to a{:}1$, $s\to b{:}10$, $a\to b{:}-8$, $a\to t{:}9$, $b\to t{:}2$, $s\to t{:}15$. Since one weight is negative, Dijkstra's algorithm cannot be trusted here. What is the TRUE shortest path length from $s$ to $t$, found by checking every route directly?