Fewest Refuels

Difficulty: 2000 5000 ms 128 MB

greedy heap

A car starts with S fuel (1 fuel = 1 km) and must reach position TARGET. Fuel stations sit at given positions, each offering a fixed amount. Print the minimum number of stops needed, or −1 if unreachable.

Input: the first line has TARGET, S and N; each of the next N lines has a station's position and fuel.

Output: the minimum number of refuel stops, or −1.

Constraints:

• 1 ≤ TARGET ≤ 10⁹
• 0 ≤ S ≤ TARGET
• 0 ≤ N ≤ 10⁵

Sample Tests

100 10 4
10 60
20 30
30 30
60 40

2

5 5 0

0

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