Okay. So, first thing is why and where Dijkstra is failing in current graph.
answer of where is for V. The correct shortest path cost for V is 2. P-->Q-->T-->U-->W-->V.
If the vertices are relaxed in this manner then surely the cost for V will come 2 and that is correct only.
Answer of Why is because we are relaxing S first before W.
Why we are doing that ? because the cost for S is 9 and for W is 13.
As now we are changing edge cost of Q-->S,there are two ways to solve it.
1st--> ensure that W is relaxed first. For that we have to make cost of S as 14, and for that Q-->S should be greater then 12 and less than or equal to 20. // 8 values.
2nd--> even if we relax S first , correct answer for V comes.
If we put Q-->S as -1, then cost for V will be 2. And for all values less than -1, cost will be evaluated correctly.// 20 values
So, total values are 28 (-20,-19,.....,-2,-1,13,14....19,20).