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## 문제

Basri, Lim and Rama are taking a road trip during their semester break. But the odometer in their car is broken, so they don't know how many miles they have driven. Fortunately, Bill has a working stopwatch, so they can record their speed and the total time they have driven. Unfortunately, their record keeping strategy is a little odd, so they need help computing the total distance driven. You are to write a program to do this computation.

For example, if their log shows

Speed (miles/hour) Total elapsed time in hours
20 2
30 6
10 7

this means they drove 2 hours at 20 miles per hour, then 6−2 = 4 hours at 30 miles per hour, then 7−6=1 hour at 10 miles per hour. The distance driven is then (2)(20) + (4)(30) + (1)(10) = 40 + 120 + 10 = 170 miles. Note that the total elapsed time is always since the beginning of the trip, not since the previous entry in their log.

## 입력

The input consists of one or more data sets. Each set starts with a line containing an integer n, (1 ≤ n ≤ 10), followed by n pairs of values, one pair per line. The first value in a pair, s, is the speed in miles per hour and the second value, t, is the total elapsed time. Both s and t are integers, (1 ≤ s ≤ 90) and (1 ≤ t ≤ 12).  The values for t are always in strictly increasing order. A value of −1 for n signals the end of the input.

## 출력

For each input data set, print the distance driven, followed by a space, followed by the word "miles".

## 예제 입력 1

3
20 2
30 6
10 7
2
60 1
30 5
4
15 1
25 2
30 3
10 5
-1


## 예제 출력 1

170 miles
180 miles
90 miles


## 출처

• 잘못된 데이터를 찾은 사람: sgchoi5