Sometimes, the schools use weighted average instead of calculating the standard average. In weighted average the course averages are weighted according to different criteria; like the number of hours in a week. If a lesson has one hour per week and student takes very low grade from that, it will not effect the general average much.

Weighted average is mostly calculated like this:

• All course averages are multiplied (weighted) with the number of course hours per week
• The results are summed
• The sum is divided by the number of total course hours per week

Write a program that is going to read the course averages and the number of hours per week from a file and then calculate the weighted average.

Input specification
The first line has a number (n) which shows the number of lessons where 1 ≤ n ≤ 10. The following n lines contain two integer numbers each (courseAve and numHours), where 1 ≤ courseAve ≤ 10 and 1 ≤ numHours ≤ 5.

Output specification
Show one floating number with two digits precision that represent the weighted average of the student.

Sample Input:
  3
  10 4
  10 3
  6 1


Sample Output:
  9.5

Output Explanation:
   ave = ((10 * 4) + (10 * 3) + (6 * 1)) / 8 = 9.5