/*

@theme:hdu 4310

@writer:pprp

@declare:简单的贪心算法 将攻击力/血量最高的敌人先进攻下来就行了

@date:2017/8/26

*/

#include <bits/stdc++.h>

using namespace std;

class enemy

{

public:

    double dps;

    double hp;

} emy[];

struct cmp

{

    bool operator()(const enemy& a, const enemy&b)

    {

        return a.dps/a.hp > b.dps/b.hp;

    }

};

int main()

{

    int n;

    while(cin >> n && n >=  && n <= )

    {

        double ans = ;

        double sum_dps = ;

        for(int i =  ; i < n ; i++)

        {

            cin >> emy[i].dps >> emy[i].hp;

            sum_dps += emy[i].dps;

        }

        sort(emy, emy + n,cmp());

//        for(int i = 0 ; i < n ;i++)

//        {

//              cout << emy[i].dps << " " << emy[i].hp << endl;

//        }

        for(int i =  ; i < n ; i++)

        {

            if(i == )

            {

                ans += emy[].hp * sum_dps;

            }

            else

            {

                sum_dps -= emy[i-].dps;

                ans += emy[i].hp * sum_dps;

            }

        }

        cout << ans << endl;

    }

    return ;

}