BOJ - 4635 - Speed Limit

import sys

def solution():
    while True:
        N = int(sys.stdin.readline())
        if N == -1: return

        tLast = 0
        ret = 0
        for _ in range(N):
            s, t = map(int, sys.stdin.readline().split())
            ret += (t - tLast) * s
            tLast = t
        print("%d miles"%ret)


solution()

https://www.acmicpc.net/problem/4635