A country consists of cities connected by bidirectional roads. Each road has a fuel requirement equal to its weight.

You are given:

• An array arr, where arr[i] denotes the fuel price per unit in city i.
• A road network connecting cities.
• A source city A.
• A destination city B.

Your truck has:

• Infinite fuel tank capacity.
• Ability to buy any amount of fuel at any city.

Find the minimum cost required to travel from city A to city B. If no path exists, return -1.

Example

Fuel Prices: arr = [9, 11, 3, 2, 10]

Suppose the optimal route is: 3 → 5 → 1 → 4 → 2

Strategy:

1. Buy fuel at City 3 (price = 3) sufficient to reach City 4.
2. At City 4 (price = 2), purchase remaining fuel needed to reach destination.
3. Since fuel can be stored indefinitely, buying more at cheaper cities reduces overall cost.

Output: 27