#include<bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
const ll maxn = 1e5 + 3 , inf = (ll)1e6;
const ll mod = 1e9 + 7;
int n;
ld eps = 1e-5;
ld a[maxn] , b[maxn];
bool ss(ld i , ld j)
{
return i > j;
}
bool check(ld mid)
{
int p1 = 1 , p2 = 1;
ld so1 = 0 , so2 = 0;
while(so1 < mid || so2 < mid)
{
while(so1 < mid)
{
if(p1 > n) return false;
so1 += a[p1] - 1;
so2 -= 1;
p1++;
}
while(so2 < mid)
{
if(p2 > n) return false;
so2 += b[p2] - 1;
so1 -= 1;
p2++;
}
}
return true;
}
ld solve()
{
ld lo = 0 , hi = inf , kq = 0;
while(lo + eps <= hi)
{
ld mid = (lo + hi) / 2.0;
if(check(mid))
{
lo = mid;
kq = max(kq , mid);
}
else hi = mid;
}
return kq;
}
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
cin>>n;
for(int i = 1 ; i <= n ; ++i)
cin>>a[i]>>b[i];
sort(a + 1 , a + n + 1 , ss);
sort(b + 1 , b + n + 1 , ss);
cout<<fixed<<setprecision(4)<<solve();
}
