#include<bits/stdc++.h>

using namespace std;

signed main(){
	int n;
	double m;
	m=0.0;
	cin>>n;
	for(int i=0;i<n;i++){
		double a;int b;
		cin>>a>>b;
		m+=a*b*1.0*0.85;
	}
	
	cout<<fixed<<setprecision(1)<<m*10/10.0;
	return 0;
}