how to iterate through each set

[Tom_chicollegeboy]

I am begginer in learning Python language.

My assignment is to iterate through each set and display results. this
is text of assignment:

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:

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.
You will implement a function speed in module speed.py that will help
Bill and Ted compute the distance they traveled on several road trips.


The input will come from file speed.in and consists of one or more
data sets where each data set represents a separate road trip. 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.

For each input set, output on the screen the distance driven, followed
by a space, followed by the word "miles". The output for input file
speed.in should be:



170 miles
180 miles
90 miles

this is speed.in file used for input:
3
20 2
30 6
10 7
2
60 1
30 5
-1

Here is my code:

def speed():
    infile = open('speed.in', 'r')
    line = infile.readline()
    read = int(line)
    print line
    i = 0
    t0 = 0
    v = 0

    while i<read:
        line = infile.readline()
        list = line.split()
        s = int(list[0])
        t1 = int(list[1])
        t = t1- t0
        v += s*t
        i+=1
        t0 = t1
    print v

It works only for first set. How should I implement another solution
that would move program to read next set? I think I should implement
nested while look, with first that reads until -1 and under it another
while loop that reads n times sets. But I have no idea how. Can you
help me?