#include <bits/stdc++.h>
using namespace std;

#define FOR(i, a, b) for(int i = (a); i < (b); i++)
#define RFOR(i, a, b) for(int i = (a) - 1; i >= (b); i--)
#define SZ(a) int(a.size())
#define ALL(a) a.begin(), a.end()
#define PB push_back
#define MP make_pair
#define F first
#define S second

typedef long long LL;
typedef vector<int> VI;
typedef vector<LL> VL;
typedef pair<int, int> PII;
typedef pair<LL, LL> PLL;
typedef double db;

LL calcD(int n, vector<PLL> as)
{
	LL v = 0;
	LL ans = 0;
	
	for(auto [a, t] : as)
	{
		ans += 2 * v * t + a * t * t;
		v += a * t;
	}
	return ans;
}



int main()
{
	ios::sync_with_stdio(0); 
	cin.tie(0);

	int n;
	cin >> n;
	
	vector<PLL> as(n);
	FOR(i, 0, n)
		cin >> as[i].F >> as[i].S;
	//FOR(i, 0, n)
	//	cin >> as[i].S;
	
	LL d0 = calcD(n, as);
	sort(ALL(as), greater());
	LL d1 = calcD(n, as);
	
	//cerr << d0 << " " << d1 << "\n";
	d1 -= d0;
	
	cout << d1 / 2 << "." << (d1 % 2 == 1 ? 5 : 0) << "\n";
	
	
	
	
	
	

	return 0;
}