#include <cstdio>
#include <iostream>
#include <algorithm>
#include <cmath>
#include <string>
#include <vector>
#include <set>
#include <map>
#include <deque>
#include <queue>
#include <list>
#include <stack>
#include <cstring>
#include <cstdlib>
#include <complex>
#include <ctime>
#include <bitset>
#include <iomanip>
#include <sstream>
using namespace std;
#define sz(v) ((int)((v).size()))
#define all(v) (v).begin(),(v).end()
typedef long long ll;
typedef pair<int,int> ii;

int n;
vector<pair<ll,int>> a, b;
ll bestAns;

void rec(int pos, ll curA, ll curB, ll curAns) {
    if (pos == n) {
        //cerr << "========" << endl;
        //for (auto x : a) cerr << x << " "; cerr << endl;
        //for (auto x : b) cerr << x << " "; cerr << ", ans = " << curAns << endl;
        bestAns = max(bestAns, curAns);
        return;
    }
    for (int i = pos; i < n; i++) {
        swap(b[i], b[pos]);

        ll L = max(curA, curB);
        ll R = min(curA + a[pos].first, curB + b[pos].first);
        ll add = max(0LL, R - L);
        if (a[pos].second != b[pos].second) add = 0;
        rec(pos + 1, curA + a[pos].first, curB + b[pos].first, curAns + add);

        swap(b[i], b[pos]);
    }
}

void solve() {
    while (cin >> n) {
        a.resize(n);
        b.resize(n);
        for (int i = 0; i < n; i++) {
            cin >> a[i].first >> b[i].first;
            a[i].second = b[i].second = i;
        }

        ll sumA = 0, sumB = 0;
        for (int i = 0; i < n; i++) {
            sumA += a[i].first;
            sumB += b[i].first;
        }

        for (auto& x : a) x.first *= sumB;
        for (auto& x : b) x.first *= sumA;

        bestAns = -1;
        for (int shift = 0; shift < n; shift++) {
            rec(0, 0, 0, 0);
            reverse(all(a));
            reverse(all(b));

            rec(0, 0, 0, 0);
            reverse(all(a));
            reverse(all(b));

            vector<pair<ll,int>> newA;
            for (int i = 1; i < n; i++) newA.push_back(a[i]);
            for (int i = 0; i < 1; i++) newA.push_back(a[i]);
            a = newA;
        }

        cout << 1.0 * bestAns / (sumA * sumB) << endl;
    }
}

int main() {
    ios::sync_with_stdio(0); cin.tie(0); cout.tie(0); cout.precision(15); cout.setf(ios::fixed);
    srand(322);

    /*for (int n = 3; n <= 64; n++) {
        cout << n << ' ';
        auto v = solve(n);
        cout << sz(v) << endl;
    }
    cout << "OK" << endl;
    return 0;*/

    //int T;
    //cin >> T;
    int T = 1;
    while (T--) {
        //cerr << "===== test started =====" << endl;
        solve();
    }
}