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

#define sz(x) (int)(x).size()
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define pb push_back
#define x first
#define y second
#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--)

typedef long long ll;
typedef double db;
typedef long double LD;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef pair<db, db> pdd;

#ifdef ONPC
mt19937 rnd(228);
#endif


int solve()
{
	int n;
	if (!(cin >> n))
		return 1;
		
	vector<pll> a(n);
	FOR (i, 0, n)
		cin >> a[i].x >> a[i].y;
	
	ll base = 0, best = 0;
	ll ac = 0;
	FOR (i, 0, n)
	{
		ll st = ac + a[i].x;
		ll en = st + a[i].x * (a[i].y - 1);
		//cerr << "st = " << st << " en = " << en << '\n';
		assert(((st + en) * a[i].y) % 2 == 0);
		ll curr = (st + en) * a[i].y / 2;
		//cerr << "ac = " << ac;
		//cerr << " curr = " << curr << '\n';
		base += curr;
		ac += a[i].x * a[i].y;
	}
	//cerr << "base = " << base << '\n';
	sort(rall(a));
	ac = 0;
	FOR (i, 0, n)
	{
		ll st = ac + a[i].x;
		ll en = st + a[i].x * (a[i].y - 1);
		//cerr << "st = " << st << " en = " << en << '\n';
		assert(((st + en) * a[i].y) % 2 == 0);
		ll curr = (st + en) * a[i].y / 2;
		//cerr << "ac = " << ac;
		//cerr << " curr = " << curr << '\n';
		best += curr;
		ac += a[i].x * a[i].y;
	}
	//cerr << "best = " << best << '\n';
	
	ll ans = best - base;
	cout << fixed << setprecision(1) << (db)ans << '\n';
	
	return 0;	
}

int32_t main()
{
	ios::sync_with_stdio(0);
	cin.tie(0);
	
	int TET = 1e9;
	//cin >> TET;
	for (int i = 1; i <= TET; i++)
	{
		if (solve())
			break;
			
		#ifdef ONPC
			cerr << "_________________________________\n";
		#endif
	}
	#ifdef ONPC
		cerr << "\nfinished in " << clock() * 1.0 / CLOCKS_PER_SEC << " sec\n";
	#endif
	return 0;
}