acmsguru 

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114. Telecasting station time limit per test: 0.25 sec.
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 citizens displeasure is equal to product of citizens amount in it by distance between city and TVstation. Find such point on Ox axis for station so that sum of displeasures of all cities is minimal.
Input Input begins from line with integer positive number N (0<N<15000) – amount of cities in Berland. Following N pairs (X, P) describes cities (0<X, P<50000), where X is a coordinate of city and P is an amount of citizens. All numbers separated by whitespace(s).
Output Write the best position for TVstation with accuracy 10^{5}^{. }
Sample Input 4 1 3 2 1 5 2 6 2
Sample Output 3.00000  

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