114. Telecasting station

114. Telecasting station

time limit per test: 0.25 sec.
memory limit per test: 4096 KB

Every city in Berland is situated on Ox axis. The government of the country decided to build new telecasting station. After many experiments Berland scientists came to a conclusion that in any city citizensdispleasure is equal to product of citizens amount in it by distance between city and TV-station. Find such point on Ox axis for station so that sum ofdispleasures of all cities is minimal.

Input

Input begins from line with integer positive number N (0<N<15000) – amount of cities in Berland. FollowingN pairs(X, P) describes cities (0<X, P<50000), whereX is a coordinate of city andP is an amount of citizens. All numbers separated by whitespace(s).

Output

Write the best position for TV-station with accuracy 10^-5.

Sample Input

41 32 15 26 2

Sample Output

3.00000

3------1-----x-----------------------2-------2             在x点建一个电视塔，使所有市民的失望值和最小

1      2                            5       6             （失望值=x到该城市的距离*该城市居民数量）题解