#### Start

2018-06-22 16:00 UTC

## DHS CompClub Practice 1

#### End

2018-06-29 16:00 UTC
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# Problem ASpeed Limit

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 $20$ $2$ $30$ $6$ $10$ $7$

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 $2 \cdot 20 + 4 \cdot 30 + 1 \cdot 10 = 40 + 120 + 10 = 170$ 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 (at most $10$). Each set starts with a line containing an integer $n$, $1 \le n \le 10$, 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, $1 \le s \le 90$ and $1 \le t \le 12$. 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 1 Sample Output 1
3
20 2
30 6
10 7
2
60 1
30 5
4
15 1
25 2
30 3
10 5
-1

170 miles
180 miles
90 miles