Problem C: Trip

## Problem C: Trip

Time Limit: 1 Sec  Memory Limit: 128 MB
Submit: 37  Solved: 12
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## Description

Pisces is going on a long trip. Initially, he takes $$P$$ units of food with him, and the total length of the trip is $$L$$ meters. To make himself energetic, Pisces consumes $$1$$ unit of food when he goes through $$1$$ meter. Besides, there are $$N$$ villages along the trip, and in the $$i$$th of them Pisces could supplement $$a_i$$ unit of food. Now, Pisces would like to find out what is the minimum number of times that he needs to supplement to reach the destination.

## Input

The first line contains a single integer $$N(1\leq N\leq 10^4)$$, which is the number of villages.

Each of the following $$N$$ lines contains $$2$$ integers, where integers in the $$i$$th line represents the distance from the $$i$$th village to the destination and the amount of food that Pisces can get in this village.

The last line contains $$2$$ integers $$L(1\leq L\leq 10^6)$$ and $$P(1\leq P\leq 10^6)$$.

## Output

Print the minimum number of times in order to reach the destination. If Pisces cannot reach the destination anyway, print "-1" (without quotes).

## Sample Input

4
4 4
5 2
11 5
15 10
25 10

## Sample Output

2

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