Problem2403--Speed Limit

### 2403: Speed Limit

Time Limit: 1 Sec  Memory Limit: 128 MB
Submit: 3  Solved: 3
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#### Description

Bill and Ted are taking a road trip. 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 in miles per hour Total elapsed time in hours

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.

#### Input

The input consists of one or more data sets. Each set starts with a line containing an integer n, 1 d n d 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 d s d 90 and 1 d t d 12. The values for
t are always in strictly increasing order. A value of −1 for n signals the end of the input.

#### Output

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

#### Sample Input Copy

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

#### Sample Output Copy

170 miles
180 miles
90 miles