<p>A car travels from a starting position to a destination which is <code>target</code> miles east of the starting position.</p>

<p>There are gas stations along the way. The gas stations are represented as an array <code>stations</code> where <code>stations[i] = [position<sub>i</sub>, fuel<sub>i</sub>]</code> indicates that the <code>i<sup>th</sup></code> gas station is <code>position<sub>i</sub></code> miles east of the starting position and has <code>fuel<sub>i</sub></code> liters of gas.</p>

<p>The car starts with an infinite tank of gas, which initially has <code>startFuel</code> liters of fuel in it. It uses one liter of gas per one mile that it drives. When the car reaches a gas station, it may stop and refuel, transferring all the gas from the station into the car.</p>

<p>Return <em>the minimum number of refueling stops the car must make in order to reach its destination</em>. If it cannot reach the destination, return <code>-1</code>.</p>

<p>Note that if the car reaches a gas station with <code>0</code> fuel left, the car can still refuel there. If the car reaches the destination with <code>0</code> fuel left, it is still considered to have arrived.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> target = 1, startFuel = 1, stations = []
<strong>Output:</strong> 0
<strong>Explanation:</strong> We can reach the target without refueling.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> target = 100, startFuel = 1, stations = [[10,100]]
<strong>Output:</strong> -1
<strong>Explanation:</strong> We can not reach the target (or even the first gas station).
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> target = 100, startFuel = 10, stations = [[10,60],[20,30],[30,30],[60,40]]
<strong>Output:</strong> 2
<strong>Explanation:</strong> We start with 10 liters of fuel.
We drive to position 10, expending 10 liters of fuel.  We refuel from 0 liters to 60 liters of gas.
Then, we drive from position 10 to position 60 (expending 50 liters of fuel),
and refuel from 10 liters to 50 liters of gas.  We then drive to and reach the target.
We made 2 refueling stops along the way, so we return 2.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= target, startFuel &lt;= 10<sup>9</sup></code></li>
	<li><code>0 &lt;= stations.length &lt;= 500</code></li>
	<li><code>1 &lt;= position<sub>i</sub> &lt; position<sub>i+1</sub> &lt; target</code></li>
	<li><code>1 &lt;= fuel<sub>i</sub> &lt; 10<sup>9</sup></code></li>
</ul>

class Solution:
    def minRefuelStops(self, target: int, startFuel: int, stations: List[List[int]]) -> int:
        # Max heap to store fuel available at gas stations we pass
        max_heap = []
        # Number of refueling stops
        refuels = 0
        # Current fuel in the tank
        current_fuel = startFuel
        # Previous position to calculate fuel usage
        prev_position = 0
        
        # Go through all stations and finally the target
        for position, fuel in stations + [[target, 0]]:
            # Reduce the current fuel based on distance traveled
            current_fuel -= (position - prev_position)
            
            # While current fuel is not enough to reach this station, refuel from previous stations
            while max_heap and current_fuel < 0:
                # Refuel with the max available fuel in the heap
                current_fuel += -heapq.heappop(max_heap)
                refuels += 1
            
            # If still not enough fuel to reach this station, return -1
            if current_fuel < 0:
                return -1
            
            # Add this station's fuel to the heap (negative because Python has min heap by default)
            heapq.heappush(max_heap, -fuel)
            
            # Update previous position
            prev_position = position
        
        return refuels

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