### Speed Limit

#### Problem 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 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.

#### Input

*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.

#### Output

#### Sample Input

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

#### Sample Output

170 miles 180 miles 90 miles