#include <iostream>
#include <map>
#include <vector>
#include <algorithm>
#include <cmath>
#include <iomanip>

int main()
{
    int n;
    std::cin >> n;
    std::map<int, std::vector<int> > gears;
    for(int i=0; i<n; i++)
    {
        int s, c;
        std::cin >> s >> c;
        gears[s].push_back(c);
    }
    
    double ans = 0;
    
    for(auto &it : gears)
    {
        std::sort(it.second.begin(), it.second.end());
        int ngears = it.second.size();
        for(int i=0; i < ngears/2; i++)
        {
            ans += std::log(it.second[ngears-1-i]) - std::log(it.second[i]);
        }
    }
    
    std::cout << std::setprecision(30) << ans << std::endl;
}