#include<algorithm>

............

sort(arr+1,arr+1+n);

............

#include<fstream>

#include<stdio.h>

using namespace std;

ifstream fin("dog.in");

ofstream fout("dog.out");

int n;

double price[105],off[105];

int main()

{

 double resault;

 char *chresault;

 fin>>n;

 for(int i=1;i<=n;i++)

 {

 fin>>price[i]>>off[i];

 }

 for(int i=1;i<=n;i++)

 {

 resault+=price[i]*off[i]/10;

 }

 sprintf(chresault,"%.2f",resault);

 fout<<chresault;

 return 0;

}