## 题目描述

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.

## 输入

The input consists of one or more data sets. Each set starts with a line containing an integer 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.

## 输出

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

Example input:

Example output:

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

## 样例输入

```
暂无
```

## 样例输出

```
暂无
```

## 参考代码

```
#include<stdio.h>
int main()
{
int n=0;
int i=0;
int data[11][2]={0,0};
int miles=0;
while(1)
{
scanf("%d",&n);
if(n==-1)
return 0;
miles=0;
for (i=1;i<=n;i++)
{
scanf("%d %d",&data[i][0],&data[i][1]);
miles+=data[i][0]*(data[i][1]-data[i-1][1]);
}
printf("%d milesn",miles);
}
return 0;
}
```

## 解析

暂无