//2008C Dennis
#include <iostream>
#include <iomanip>
using namespace std;

int main()
{
	int n;
	while (cin>>n && n!=0)
	{
		double v,t,distance=0,time=0;
		for (int i=0;i<n;i++) 
		{
			cin>>v>>t;
			distance+=v*t;
			time+=t;
		}
		cout<<fixed<<setprecision(2)<<distance/time<<endl;		
	}

	return 0;
}