#include <bits/stdc++.h>
using namespace std;
int main()
{
int n;
cin >> n;
vector<double> a;
vector<double> b;
for(int i = 0; i < n; i++)
{
double x, y;
cin >> x >> y;
a.push_back(x);
b.push_back(y);
}
sort(a.begin(), a.end());
sort(b.begin(), b.end());
reverse(a.begin(), a.end());
reverse(b.begin(), b.end());
double odp = 0;
double suma1 = 0;
double suma2 = 0;
int it = 0;
for(int i = 0; i < n; i++)
{
suma1 += a[i];
while(it < n && min(suma1, suma2) - (double)i - (double)1 - (double)it < min(suma1, suma2 + b[it]) - (double)i - (double)1 - (double)it - (double)1)
{
suma2 += b[it];
it++;
}
odp = max(odp, min(suma1, suma2) - (double)i - (double)1 - (double)it);
}
printf("%.4lf",(double)odp);
return 0;
}
