**Thornhill Computer Club 2024 - April Contest - Problem 1**

An odd country taxes customers in stores for buying certain amounts of items, in order to encourage bulk buying:

Number different of items |
Tax cost |
---|---|

1 only | $2 |

2 or 3 | $1 |

4 or 5 | $0.5 |

6 or more | $0 |

Given the number of items purchased by a single customer and each of their prices, determine the final cost to buy the items.

**Input Specification**

The first line will contain an integer representing the number of different items that the customer will buy.

The next lines will consist of 2 space-separated integers, , the cost of the item, and , the amount of the item that the customer will buy.

**Output Specification**

On a single line, output the cost of the entire purchase.

**Sample Input 1**

```
2
5 8
8 1
```

**Sample Output 1**

`49.0`

**Explanation for Sample Output 1**

The customer wants to buy 8 $5 items, and they also want to buy 1 $8 item. This adds up to $40 + $8 = $48. Additionally, the customer wants to buy a total 2 items, so the store will charge $1 on top, so the final cost is $49.

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