Graph Algorithms · Medium problem

A weighted graph has vertices $s,a,b,c,t$ and edges $sa{:}4, sb{:}7, ab{:}2, at{:}9, bt{:}3, ac{:}5, ct{:}2$. If the toll on road $bt$ rises from $3$ to $8$, by how much does the shortest-path distance from $s$ to $t$ increase?