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.

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.

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 |