Graph Algorithms · Medium problem

A weighted graph has vertices $s,a,b,c,d,t$ and edges $sa{:}4, sb{:}2, ac{:}3, bc{:}6, bd{:}5, ct{:}5, dt{:}7$. A courier, without checking for a cheaper option, takes the route $s \to b \to d \to t$. By how much more expensive is this than the TRUE shortest route from $s$ to $t$?