#include<bits/stdc++.h>
using namespace std;
int main(){
	long double n;
	cin>>n;
	long double ans=0,a,b;
	for(int i=1;i<=n;i++){
		cin>>a>>b;
		ans+=a*b*8.5;
	}
	printf("%.1lf", double(int(ans)) / 10.0);
	return 0;
}

https://www.luogu.com.cn/record/126343970