Description

You are playing a game. The goal of this game is to kill the monster when you are still alive (HP > 0, HP = health point). The monster is dead if and only if its HP <= 0.

This game consists of rounds, and you go first in each round.

You have 3 options in each round:

1. ATTACK. This will deduct the monster's HP. If your damage point is p, then you can deduct p monster's HP.
2. HEAL. You can recover some of your HP, but it can't exceed your max HP.
3. Level-up. Level-up might enhance the effect of attacking or healing, but if you are not lucky enough, sometimes it will deduct the effect of attacking and healing. Thus, choosing to level-up is not always a good choice. If you have reached max level, taking this action makes no effect.

In each round, the monster will attack you once with a fixed damage point.

You start the game with full HP and level 1. What is the minimun number of rounds you need to kill the monster?

Input

First line contains 4 numbers L, HP, MHP, MDMG.

• L = max level
• HP = your max HP
• MHP = monster's HP in the beginning of game
• MDMG = monster's damage point

The next L lines describes each level, i-th line of them describes level i. Each of them consists of two numbers, DMG and HL.

• DMG = your damage point when you use ATTACK at level i
• HL = amount of HP you can recover when you use HEAL at level i

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Limits:

• L <= 15
• HP <= 300
• MHP <= 400
• MDMG <= 10000
• DMG <= 10000
• HL <= 50

Output

Print a single integer, denoting the mininum number of steps you need to kill the monster.

If you are unable to beat the monster, print -1.

Sample Input  Download

Sample Output  Download

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