# Problem G

Speed 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 |